Olivier,
I'm the RV-8 beta tester for the RV-8. I've had my ems in a couple of locations through the last year. I've settled on mounting it on a bracket on top of my radio stack, which is on the left side of my panel.
Really, the location isn't critical. You could mount it on the aft side of your baggage compartment, or any place you can find to mount it securely.
In my airplane, the engine sensor cables were already installed, and that more or less limited where my EMS could go.
The only restriction that I know of, is that the EMS must not be on the engine side of the firewall.
The unit is small and you will not need any type of access to it, so you're pretty much able to put it anywhere it fits up there.
Rich Wilson