its not the rear brakes that are the problem, its the front 312mm(12.3") that fit no smaller than a 16inch. the rears are just a vented 256mm (10.1)rotors 15s will clear the back.
normal mk4s are 288mm(11.3) and 232mm rear(9.1")
the R32 only fits certain 17inch wheels because of the large dual piston calipers and massive 334mm(13.15") front rotors