having done 4 trackdays in the S3 I feel the only place its let down is a slight reluctance on the intial turn in, once turned the grip is very good. I find it pretty forgiving and predictable. Once on the power if the front looses grip, it sends some power to the rear which seems to help turn the car then the power is transferred back to the front to sort of pull you out of the corner, something like that
they struggle on the brakes too because of the extra lard
I've had a couple of modded GTI's so have driven both although not on the track in the GTI, but of lot of the opinions are from people who probably haven't driven an S3/Golf R on track
the TTRS runs a very similar system and they are pretty capable