I've been driving with ABS off for about 18 months now and I use a G27. It probably took me a few weeks to really figure it out, but in the end, I think it came down to two very important factors.
1: Brake Balance - The brake balancer was once an optional modification on past GT games. PD have set the balancer to 5/5 by default, just as cars have ABS set to ON by default. Switch the ABS off, switch the Brake Balancer off. I turn both front and rear down to zero...
Why?
Drive a car with pathetic brakes. Take for example, a 1970 Dodge Charger R/T and turn ABS off. Take it for a spin (literally), on Comfort Hard tyres and you'll figure out that the brake balance setting of 5/5 is way too high for a car with drum brakes. Turn it down to Zero and all of a sudden, the car feels a great deal more realistic. Braking distances are appropriately longer and there's a great deal more feel in the brake pedal. You can still get lockups, but it's no longer an issue of more than 25% brake pressure, it's more an issue as to how long you're on the brakes for.
The same applies to just about any car I've driven. 0/0 front and rear and you'll find that the pedals have much more feel and you can modulate the braking a great deal more. With almost any car in real life, you can generally apply the brakes very firmly at high speed without lockup, but you need to come off the brakes gradually as the speed washes away in order to avoid flat-spotting your tyres. A lot of novice drivers tend to apply brakes gently at high speed and then increase pressure (as panic builds that they're not slowing down enough), which leads to the inevitable spin (or armco barrier in some cases).
2: Heel and toe - Downshifting under braking will naturally lead to torque being fed through the rear wheels in all cars except FF machines. This torque shunt mismatches the speed of the road with the rear wheels and leads to the back tyres locking up. Heel and toeing is a crucial element to avoid this when racing without ABS.
That's my two bob anyways. My lap times are now just as quick, if not quicker than they ever were prior to ABS being used and having the aid switched off is my preferred method of driving. For those of you who are keen, definitely give it a try, you'll find a deeper layer to the physics engine that you never suspected was there before.
This is a great thread BTW, I have an old DFGT for use when mates come around and I love the idea of sponges under the pedals. Will definitely give them a try and see what the boys say!
Cheerskis