Regarding ABS:
I have been driving with ABS=1 since I started playing GT5P (about 2 months). I assumed that it would just give me that little insurance against locking up the tires.
Yesterday I was reading the GT4 tuning guide and it said that ABS always hurts braking performance, so I tried turning it off (700PP Suzuka). I was stunned. After I got the bias dialed in I found my braking distances were dramatically reduced. I would hit the brakes at the "normal" spot and I would almost come to a complete stop before I even got to my turn-in point.
And then I also found that suddenly I had dramatic lift-off oversteer. No brakes applied, just lifting off the throttle. This had never been a problem before. The ABS should have no effect on lift-off oversteer, but somehow it does (at least with my car/setup). At first I didn't believe it, so I turned the ABS back to 1 and viola, no more lift-off oversteer. I even checked to make sure I hadn't turned on ASM by accident. So I had to dial in some rear toe to reduce the lift-off oversteer. I still don't have it licked completely, but it's better.
The moral of this story is: ABS=1 does more than you think it does.