Yes, you could put a 95 ecu in a 91...
There are pinout changes that need to be made though, and it gets kinda involved.
After all that work though, you would still have a STOCK ecu that needed to be tuned.
Why not just tune the ecu you have now?
The guys telling you to swap ecu's either don't want to help you (they are already busy), or they want to charge you for performing the conversion... big $$$
Tune your current ecu... that is the smartest choice.
BTW,
You don't have a 91 Cobra ecu.
Only 93 Cobras were different with respect to the ecu. Canada had a 'Cobra' available, but it was basically a badged Mustang.
good luck,
jason