I've talked to folks at our FSDO and ACO and I think they understand the problem. But, they have guidance that tells them what has to be done. I've talked to people way up the food chain and they get it and say it has to change. Some middle manager who has the responsibility to change the guidance but don't feel they have the authority is the fly in the ointment. I think if you're going to change the way the FAA works, it has to be from the top down or the result of a major fatal accident
Years ago I was on the ASTM committee (as a private individual) that was working on the new means of compliance standards to go along with the new Part 23. On that committee (and officially in an advisory-only, non-voting position, but we all know who held the reins) were regulators from EASA and the FAA, and they provided input.
I learned a couple of things.
First, while the Europeans tend to be more strict, proactive, and controlling in their regulation, and don't allow as much freedom as we have in the US (with operations, E-AB, etc), they are
much more open to discussing new ideas, new ways of doing things, and changing stuff that no longer makes sense or isn't working the way they want. The FAA, on the other hand, tends to be much more reactive, and tends not to regulate unless a need has been demonstrated. But, once they make up their mind, they ain't never gonna change it. Pointing out that a regulation doesn't work, is outdated, or is not having the intended effect merely results in them doubling-down on that regulation (see the new LODA requirement). One regulator stated that certain "breakpoints" in the regulations--specifically, the 61kt single-engine stall speed requirement, and the 12,500lb requirement for a type rating--might as well have been "carved into the third tablet that Moses dropped coming down the mountain". In another case, I asked why they were insisting on retaining the language from the "old" Part 23 that said nav light lenses needed to be "flameproof" but landing and taxi lights need only "not present a fire hazard", instead of making all the language consistent. The response was essentially "yes, we agree that doesn't make sense, no, we don't think it should be written like that, but it's been like that for a long time and we don't know why it was written that way to begin with, so we aren't going to change it".
Keep in mind too that in the eyes of most of the government--FAA, NASA, etc--a "small airplane" is a King Air, or Cirrus Jet, or PC-12. Light piston aircraft barely even register on their radar.
Also keep in mind that the FAA is a government bureaucracy. They move at their own pace, and big players get priority over smaller ones. Even pre-Covid I saw stuff at work that was stuck in FAA limbo, because they didn't have the manpower to get it done faster and they weren't about to put in the overtime that those of us in industry were subject to. I don't know how to check, but I'd suspect Dynon has to work with the same ACO that Boeing does (being in the Seattle area) and thus they're sort of stuck in line behind them.
Finally, unless it just has absolutely no other option, the FAA is going to try and cram anything new into an existing box, even if the new thing only bears the most passing resemblance to the other things in that box. Many new technologies, such as electronic engine controls, were first applied at the airliner level, and the regs were written to the standard that was needed for that application. Anyone who came along later trying to apply that technology to a less critical application (like light airplanes) just got pointed to the existing regs, no matter how "overkill" they may have been.
If you have to redo the paperwork for every version you'll beat your head against the FAA forever. For example, there are Navion models A-H. Is that 8 STCs to do? No, North American designed the control rigging in 1946 and nobody after that thought there was any reason to change it. The flight characteristics didn't change radically, why would the STC or the limitations change? Perhaps there are servo mounting bracket changes between subtypes, but should that restart the STC process? Looking at the AML, even the C172 only has about half of sub-models approved for autopilot. About 1/3 of the Bonanzas approved for autopilot. Did something change so radically that it would change how the autopilot works in the other half of those airplanes? probably not.
Looking through some of the FAA's guidance material--AC23-17C, AC21.101-1B, etc.--there are lots of things that could trigger having to repeat testing on different submodels.
AC 23-33 states (emphasis mine):
The AML STC process should not be applied to those systems whose installation configuration varies significantly among various serial number aircraft of the same model, or those systems that can directly control the aircraft. For example, an AML STC would not be suitable for autopilot installations. For autopilot installations, there may be serial number specific airplane-rigging problems that have to be taken care of on a case-by-case basis.
I have also run across FAA guidance (which I cannot find at the moment, unfortunately) that says autopilot installation automatically requires an STC and can't just be done with a 337.
Now, in a sane world, and even within FAA certification processes (the two don't always overlap), you can get "credit" for certain things based on "similarity", so there are many parts of the integrated Skyview-autopilot system that they shouldn't have to put through the process again, and only the airframe-specific items (structural, handling/performance, electrical load) should have to get redone. But it also wouldn't surprise me if someone along the FAA chain decided out of ignorance to require everything be totally redone for each model.
And then, in AC 23-17C:
Alterations of increased engine horsepower (and either engine horsepower or major changes in exterior cowlings and surfaces), in part 23 airplanes, should consider the compatibility of the autopilot system with the increased horsepower. This is because the malfunction and performance tests of the autopilot are conducted with a defined amount of engine power. An increase in engine horsepower beyond 10 percent may adversely affect the autopilot system malfunctions, performance, controllability, and longitudinal stability characteristics. Therefore, flight testing may be necessary to verify the original approval of the autopilot system is still valid.
(a)The results of malfunction testing determine which flight condition is most critical. The effects of autopilot runaways are more pronounced at aft CG. Also, the phase of flight with the largest contribution to adverse conditions varies with airplane model.
(b)Airplane longitudinal stability is a factor in autopilot system malfunctions. There is an inverse relationship between engine horsepower and longitudinal stability. Although the turbine engine installations replacing reciprocating engines may be flat rated, the turbine is capable of producing increased horsepower at higher temperatures and altitudes, which could reduce longitudinal stability. Therefore, autopilot performance, especially the pitch axis hardover malfunction, should be evaluated for acceptability. This policy is also applicable to power increases on airplanes with reciprocating engines, either engine replacement or engine modifications that add a turbocharger.
(c)Performance and controllability evaluations should be considered, including the configuration of most forward CG and minimum autopilot authority. This configuration is used to show the airplane can be safely controlled by the autopilot when the control surface hinge moment is the highest and the autopilot controllability is at its lowest during corresponding longitudinal trim and airspeed changes.
I'm not super familiar with your example of the Navion, but some Google searching turned up a page with a brief history. It looks like that line, over time, went from 185hp up to 260hp, and may have a variety of auxiliary fuselage or wingtip fuel tanks and a range of gross weights. Even though the mechanics of the control system may not have changed, the FAA guidance suggests that certain flight and performance characteristics (especially around the edges of the autopilot engagement envelope) may be quite different and therefore those different conditions need to be tested. Those of us in the experimental world have the freedom to tune our autopilot parameters as we see fit and don't have to jump through all the regulatory hoops and tests that certified products (including things in an STC) have to--but then, we have no guarantees of performance and all the attendant risk falls back on us. But a certified product, in essence, comes with a guarantee that says (in essence) "if you build/install this thing, exactly per these directions, you will get this performance"--and that means they have to do the tests when there are differences that could affect said performance.
Now, as for the elephant in the room, we could sit down with a few beers and hash out whether this level of scrutiny and tightly-controlled configuration and certification is really necessary for light airplanes. The FAA claims to have addressed this with the Part 23 rewrite, but all that really seems to have done is make it easier to make new "boxes" to put things in, not reduce the testing, paperwork, and configuration control burden that certified aircraft are saddled with. Personally I think the FAA should just provide a pathway for people to irrevocably convert their certified light airplanes to an "experimental-personal" category that would make those airplanes substantially equivalent to secondhand homebuilts.