Hi,
I had the same problem with my GS - vibrations on the steering wheel while braking, in my case rotors caused the problem - changed braking pads several times - didn't help at all. New rotors are perfect even and smooth with no scratches, hollows whatsoever. I bought second hand car with second hand rotors I replaced both of them for new, sport ventilated ones B) - no problems with vibrations on the steering wheel anymore.
The car is pretty heavy - braking system - pads, rotors are very, very sensitive - everything has to be spotless - no dust, rust, grease whatsoever.
Good luck
Jarek