The wheel arches on the GTO are massive, you could smuggle five families through customs at Dover in each one and no-one would notice. With 17" rims and standard ride height it just looks stupid. I put coil over suspension on to tighten up the handling and lowered it, it also has adjustable rear camber control arms. When i bought the Rotora 355 front discs i needed something to fit over them, decent 18" rims that would clear weren't available then so i found some 19's that would fit.
I'm now in the process of finding a new set of alloys, they will be 19's again but it's an awful task finding the combination of, ones i like, that fit, are the size i want with the correct offset and don't look out of place on the car and of course aren't on 50 other GTO's. I may have to compromise somewhere.
The downside to going up in rim diameter is coming down in tyre profile, which handles better on good roads but you can feel every bump, ripple and deviation in the road surface, which over here in the good old U.K. is rather constant.
I do know of a couple of people with 20" rims on their GTO's but the tyres have the profile of a rubber band. They do look good though.
If i remember, i'll put up a phot later of mine.