Your tuner is talking about an A9L conversion. Its a fox ecu. Its not a straight plug and play setup but it can be done. You've got to move some ecu pins around on your harness and also figure out a new way to control your fans as fox bodies had mechanical fan. As for the crystal swap mod...never heard of it and wouldn't attempt it.
Its not that the 94-95 ecu is slower, its more sensitive and when tuned PROPERLY by a COMPOTENT TUNER, it will run just FINE. Honestly, if your tuner is trying to talk you into going BACKWARDS technology wise and running an older ECU just so he can tune it....I wouldn't be looking for another ECU...I'd be looking for another tuner.
<---Diablosport Chipped factory 94GT ECU tuned by Wayne's Dyno & Performance....for 2 years running