Im very impressed with this build. Especially with the full Controller spec and design. I'm working on something similar, and until now i haven't seen any one else do that (to my knowledge). While it may be hard to justify all that for the current Pi hardware, and or limitations of the systems it can emulate (i.e. not taking full advantage of the controller) I have been testing Moonlight-embedded with Launchbox (Bigbox mode) to access not only controller based steam games, but the rest of the emulators the Pi cannot handle.
Hats off to you!
This is Solid!