My $0.02:
In general: ABS is not intended to reduce stopping distance. It is intended to help the driver maintain maneuverability in a panic situation. It intentionally does NOT push the braking to the limit, because that impairs lateral grip. Just how much braking force is sacrificed in favor of maneuvering is up to the engineers who set the calibration.
SyTy specific: The application of the Corvette-spec 4WAL system to our trucks looks good on paper, but does not work well. The reason is that our front and rear wheels are linked via the viscous clutch transfer case. The 4WAL system pulses pressure to 3 zones independantly (front left, front right, and rear). PAS/GM/Kelsey-Hayes discovered, when setting up the calibration for SyTy ABS, that pulsing the rear independantly of the fronts set up a destructive force in the AWD system. So, their solution was to drop rear brake pressure to zero, and only pulse the front brakes during ABS activation! (how many FRONT WHEEL ONLY ABS systems can you recall?! There's a good reason!) Thus, with ABS activated, our front tires are now doing all the stopping. And, in order to maintain maneuverability as discussed above, the braking force has to be reduced... end result: ABS braking force is reduced so dramatically from non-abs braking foce, it feels like you just took your foot off the brake pedal.
I had an opportunity to experience an "A-B" comparison on dry pavement. My ABS stopped working when I did my LS1 rear disc conversion, because I ground too far on my left rear sensor. Most of the time, the ABS light was on, and I had only normal brakes. However, once in a while, the ABS computer would get just enough signal from the damaged sensor to turn the ABS light out.
I didn't think much about it, until one day I was pulling to a stop behind cars at a red light. Suddenly, the ABS light goes OUT, and the pedal starts pulsing (apparently, the computer was getting a signal of some sort from the damaged sensor, but it was interpreting that that sensor was not at the same speed as the other 3). I could only watch in horror as my previously-safe stopping distance was now obviously not going to be enough. Not by a wide margin. Fortunately, the ABS system must have lost the signal from the left rear, the ABS light came back ON, ABS quit, and I had barely enough distance to stop before hitting the car in front of me.
My ABS came out the next weekend.
When GM ABS systems go haywire its not good in the least bit.
