The A9P is based on slightly older code (GUF1) so there are some differences between it and the A9L (GUFB). The A9P's brother EECs that are 100% interchangeable with it are the C3W and C3W1. They are to the A9P the way all those other EECs mentioned are to the A9L. Notice the codes GUF1 and GUFB. Those are memory strategy codes. All EECs following the same strategy can be loaded with the same parameters of any other EEC of the same strategy.
The A9P code is missing some idle control logic that the A9L has. And the settings in the code that's common between the two are fairly different. It's quite possible for someone to notice the difference between an A9P and A9L, but surprisingly MOST people with manual trannys don't. In fact, the only people that notice the difference are people with automatics that replace their A9P (tuned for automatics) with an A9L (tuned for manuals). What they notice is when they put the tranny in gear, the engine could stumble or completely conk out when the tranny loads up the engine in drive/reverse. With a tuning device, this is 100% tunable for in the A9L because the A9L has ALL the code the A9P has...just different settings/parameters.
The A9T is another EEC that follows the same memory strategy as A9P. The A9T was tuned for the State Patrol 5.0L Mustangs in the late 80s and early 90s. They are rich-tuned in Closed Loop, but at WOT run a bit leaner than stock A9P. The stock 5.0L GT EECs are notorious for being overly rich at WOT to the point of loosing power. Presumably Ford made the assumption that people would put mods on the engine and to be safe, it is rich-tuned...better to be too rich than too lean. Another benefit the A9T has over the A9P is a higher spark advance at WOT and non-WOT conditions. I have no clue what mechanical differences there were between the stock GTs and the State Patrol 5.0Ls, but the tune is certainly different and obviously directed to improve performance. The only non-performance difference in the tune is a slightly higher EGR flow likely to help with NOx emissions due to the higher spark advance.
Interestingly the A9L has a similar cousin EEC that follows its strategy called the X3Z. The X3Z is the EEC used on the 93 Cobra. It is NOT a direct drop-in for the A9L because of 2 very important differences amongst 100s of minor differences. The X3Z was tuned expecting 24lb injectors and a 70mm MAF which is the main reason it isn't a direct swap. But because it follows the same strategy as the A9L, you can load an X3Z tune into an A9L using the TwEECer or Moates chip.
One of the most interesting differences about the X3Z is it supports Decel Fuel Shut-Off (DFSO) where none of the Mustang GT tunes have this enabled. What this does is turns off the injectors when the RPMs are held high at closed throttle by a manual tranny. So for instance, if you have DFSO enabled and coast down a hill in gear, the EEC will literally stop firing the injectors so you burn no fuel going down a hill. This does create additional engine braking as compared to an engine without DFSO enabled. Higher compression and stroker engines experience a fair amount of additional engine braking,
