The G20 I owned always had the typical buildup of brake dust on the front wheels...not surprising. But the GTI seems to have more buildup on the rear wheels than the front. Strange.
In your case, Eddy4, I'd be surprised if it had anything to do with the traction control.. I figured in the case of my GTI (and completely without basis in knowledge) that it was simply a matter of aerodynamics and the size and shape of the wheel openings and brake rotors.