As you can see from the image below, the lower OEM offset means the wheels sit much further out than the GTC and GT wheels. Why? the GTC & GT wheels are much wider (10-12" wide, vs ours which are 8-9" wide), so they need to sit further inward to allow the wheels not to push out past the fender/quarter panel. So not compatible, and not something that spacers could help with (would push wheels in the wrong direction).
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
