I never planned to change my rear brakes from stock but got tempted one morning last year in the For Sale section by DaveB who is something of a brake specialist. Yes my rear would occasionally get slightly twitchy under hard braking but nothing to worry about. I have AP Racing Big Brake Kit on the fronts and can't praise them highly enough.
My stock GTI rear discs were not in good shape:
Volkswagen Racing fitted my S3 rears for me and not without difficulty in undoing a spline on each side. Not a job I would consider doing myself:
I didn't know quite what to expect when I went out for a test drive. To be honest I was very concerned that I might fall victim to the placebo effect. However, the car's braking now feels very solidly even front and back and there is no sign of any twitching so far.
I had plenty of time to paint the calipers red but I think they look better left in black with my OZ Titanium colour wheels. There is volumes of space for ventilation and brake dust disposal with my OZ Alleggerita's. I didn't need to do this mod but I'm rather pleased I did.
There are quite a few other GTI/Ed30's on this forum now fitted with S3/R32 rear brakes.