The slower PCM actually has nothing to do with car performance once a tune is written. A tune is a tune, and the 96-97 simply refresh their information more slowly. If you're using Windows, go to task manager. Go to the Performance tab and then click
View > Update Speed > (High, Normal, Low).
You're not speeding up or slowing down the computer. It just changes the update interval. The difference is that the 96 works the same as the 99, engine-wise, but the refresh rate of the PCM to the tuner is slower. It is supposed that the newer PCMs can detect knock and account for it much sooner, and I believe that. But that'* about all.
GM'* switch to the 94/95 PCM didn't do much of anything. Those were years when GM was figuring out where to go next with the wiring. You know, how to make it arbitrarily more complicated.