The gt3 brakes will need at least an 18" wheel, if not a 19. Last weekend I was chatting with a gt3 owner and he said he had used 18' wheels on his car and they had rubbed the brake calipers.
I have EBC Yellow pads and ATE Blue fluid in my car, with new standard disks. The brakes are awesome and stood up very well to 80+ laps at the track last week. My stock set up was useless after 30 or so hot laps.