You don't need anything but the engine itself and it'll work just fine.I'm under the impression that if I take a complete 954 engine, TBs/injectors and all, and put it into a 929 that it will work. Is this correct? Also, how much of a performance hit will I take by not using the 954 ECU - and how much can be made up with a PC3?
You can run the 929 or 954 throttle bodies.
No idea what gains you might see from the ECU. Personally I think most of the 954's 10hp over the 929 comes from the throttle bodies. To run the ECU you will have to replace the entire harness and whatever else is wired differently - instruments, tail light, etc.