Wheels and Exhaust
Wheels: I had a set of 18 x 8 wheels made by Boyd Coddington. The pattern design is called Genesis. It's within their Gotcha series. They take about 6 weeks to make -- and they look great. I don't think getting 8.5s underneath the front of the car will be a problem at all. On the back, I'll bet you can get 10's under there, easy. I saw a street Monaro down under built by a racing fabrication shop -- and they had something like 19 x 9 Breytons up front and 20 x 10s out back. It was ridiculous!
Look at it this way, the offset on the factory wheels is 6.125. To calculate the maximum size, just get under there and make some calculations.
And don't forget, you want wheels with a little depth to them. Never been a fan of wheels where the dish and rim are on the same plane -- like the spokes on the factory ones. With a custom built wheel, you can avoid that.
Exhaust: No aftermarket kits are out that I'm aware of. Dollar for dollar, though, exhaust kits are not a very good value. You get far more bang for your buck via alternative methods, such as chip and/or software development. Look at it this way, the factory system sounds and breathes plenty good already. If you're looking for power gain, there are other ways to further ahead.
Good luck with your car. The new GTO is awesome. As you say, A BEAST! For a picture of my car with the Boyd's wheels on it, just go into the gallery and look at recent posts. My car's yellow with polished five spoke wheels.