When I hook up the mercruiser scan tool to my 502 MPI the scan only shows 75% throttle opening when I have the throttles wide open. The buttleflys are open all the way when you look at them at wide open thru the throttle body. Will this affect top end or is this just the way the computer is programmed ?

Yes, that will affect the fuel curve. The injection timing will probably never go asynchronous @ WOT. Probably just need a new TPS.

I'm thinking TPS. Try adjusting it first. You might just get lucky.

TPS.

Disconnect the throttle cable and check TPS output voltage at idle and WOT, and also slowly move from one extreme to the other and the voltmeter needle should swing smoothly with it with no jumping around or dead spots..