The gain for MP3 owners switching to a P5 ECU is that the P% ECU doesn't have the advanced timing the MP3 has. This is only beneficial in turbo/supercharging/N2O applications.
All motor cars benefit from advanced timing because it gives the crankshaft time to rotate around and get a better angle with the rod before the spark plug fires.
This can also be achieved by moving the rod pin up higher towards the top of the piston. This increases dwell time, the time the piston stays at TDC, also allowing the crank time to roatate and get a better angle.
Both advance timing and moving the pin up higher are bad for FI. If the pin is higher in a turbo car, it and the bearing stand more chance of being damaged by detonation.
Not exactly sure on the advanced timing though. Has something to do with the increased volume of air/fuel, takes longer to burn, so you need to retard the timing a bit.
SO...
P5/Protege will see 10 crank HP from an MP3 ECU, and MP3 will have a more boost friendly timing curve.
A few MP3 owners on here have P5 ECU's. Forgot the names though. I see no bad side effects to this swap. Exact same engine in all the cars, even the MSP.