If you are looking on TireRack's website, look under 2005 BMW 330 Coupe. The bolt pattern is the same and they give you about 120 more wheel choices. Just remember to keep that offset in mind!
I did more research and you will need to be in the 50mm-60mm range to fit a 9" wide rear in. I've been told by a TireRack sales guy who owned a 05 GTO that he fit a 18x9 with 265mm rear without problems, but the way these cars vary, I don't think it means anything.
Personally, if you can find a good 18x8 with a 45mm offset and 255mm rubber, that is a good compromise. Having some of the suspension issues addressed will surely help.
If you can get a proper alignment done, you should be good to go with the 255mm set-up. What do I mean by proper alignment? Call an ex-pontiac dealer (probably still carry Buick/GMC) and ask if they still have the GTO alignment tool. It is a "jig", of sorts, that mounts into the front and rear cradles and into the body. This makes sure that, not only are the front and rear suspensions in line with each other, but that the body is also centered with the suspension (these cars have been known to be shifted to one side or the other, making it difficult to fit wider rears).
I still am not 100% that a 255mm tire will fit up front, regardless of the offset. I still have more research to do. Keep me posted if you discover anything on this issue.
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