AODs are junk for performance work, unless you really have them built with a properly matched torque converter.
The T5 in the '85, if its still the original transmission, is not the best T5 to ever have been in a Mustang. '85 was the first year for the "World Class" T5 units, but some non-WC T5s apparently carried over from the previous year. So your may have either. The World Class units had 300+ ft-lb torque ratings, while the non-WC had like 285 or less. If you can find the part number, the "WC" T5 is "E5ZR-7003-FA". Over the years, the components Ford used in them improved their torque ratings and shifting reliability, but if yours is a WC and still shifts fine, I wouldn't worry about it.