Dave
The pcm that came in your car is very capable of doing what ever you would want to do for a street combo at say the level of 300rwhp or better. Any tuner who can tune Stangs other than older Foxes would tell you he can make your car do what ever is needed with your original Cobra pcm.
Many reasons not to put the older pcm in your Stang but here is just one example ................
The act in the fox is in the intake and its in the pipe just before the tb in our cars. When you put the older pcm in our cars it sees temps from the act that are much cooler than it is expecting and then that throws off all the temp related conditions the pcm tries to deal with. Lots of decisions made by the pcm are based on the act temps.
A combo at the above level would benefit form a custom tune would you not agree?
Why should you have to go to all the trouble to fix all that stuff that is incompatible with the Fox pcm in our Stang before you even get started with the custom tune.
The Fox pcm retrofit thing in our cars was an option that many found to be a benefit for them ........ However ..........
Don't forget one thing ........................
This ain't 1996 anymore .....................
Technology has come to the point of giving us more options like PMS (still can't believe they picked that name), Tweecer, and Chips by SCT and the like.
Later
Grady