I suppose you would be getting into the bigger piston area softening the pedal feel scenario. Bigger discs with the stock calipers would be better, imo, rather than MK5 rear calipers. Not necessary on a FWD MK5 anyway, just bung the endless pads in the back to compliment the fronts. The only reason S3/R32 got bigger rear brakes was because of overall vehicle weight + more weight over the back wheels. FWD doesn't have that to contend with.
MK7 R fronts with stock rears feels absolutely spot on for a road car.