Exhaust size

I do not know that it will make a difference but the larger pipe will flow a bit more.

Modern cars are set up to run best with a specific back pressure on the exhaust side. While I agree with F O R, the larger diameter pipe should flow better, you may find you need to do some other adjustments if you move away from factory spec..

Agree with both answers above. First of all 1/4" change in diameter will not make any difference that you will be able to feel. Second, exhaust changes need to be made with care so you don't waste money or mess some other system up. Do your homework. ZZ Performance would be a good place to start.

Well I believe my car has 2" on it and was thinking going to 2½" what's ur thoughts on that? What eould work best with my supercharger

2.25" vs 2.50" will flow 19% more.

My thoughts are: go to a website like ZZ Performance that specializes in all types of upgrades for the Pontiac Grand Prix. Look at the various packages available and read some of the product information and guides that are found there. Join the Grand Prix Forum and read the various threads and articles there that document the various things you can do to your car. You will also learn about lots of things you should not do, and things you can do but that won't make any difference. Unless you've got endless amounts of money to spend on this car, you want to make sure you're spending it on things that will actually improve your car. Bigger exhaust is not necessarily the first thing you should be doing.

Thank you. I do have a cold air intake and headers already so

Good start. You could also look at a smaller pulley on your supercharger. Don't get too carried away with that though or you'll create other problems. A smaller pulley may require a smaller belt and a special puller to remove the old one as well, so do your homework before ordering. One freight bill is better than 3.

I know a little bit about getting a underdrive pulley and was gonna do that but unsure on what one. I have i believe a 4" pulley on it and was thinking dropping it to a 3" any advice on that you could give me on that to help me and where to get the pulley, pulley puller and belt. I also have MSD 10.4 spark plug wires on it. Any advise is greatly appreciated.

A 3" pulley is way too small. Go to ZZ Performance website and look at what is recommended for your car. The smaller you go the more additional work you'll need to do to your car to make it work. ie. additional intake air cooling and a computer re-tune to make it run right and not overheat or set off the antiknock sensors etc. They will also have the puller if you need one. Any type of premium grade ignition wires will work fine. If the ones you have are relatively new they're probably OK. Make sure the plugs are fresh, but then again if you're going to make other changes different plugs may be recommended. You can contact ZZP directly and ask questions. They will get back to you with useful info. I've modified a Cobalt SS with them and they've been very helpful.