Thought about that myself. Actually, at certain angles, the gap over the tire on a GTO looks narrower than the front. Got the tape measure to check -- and it's an illusion.
That said, you could, in theory, install new springs or try different combinations of spring rubbers. However, by raising the car, you're going to modify the geometry of the rear suspension -- so you might need an alignment. Also, you'll be putting a slight amount of stress on the IRS universal joints -- which may cause them to wear prematurely. As anmracing pointed out, you're much better off dropping the front a half inch or so...
Good luck!