The biggest difference in exhaust on any Grand Prix (performance wise) is from the downpipe. These guys have been making a better, higher flowing downpipe for the GXP for a couple of years now. Check the website out. They dont' have muffs or the remainder of the cat back, but really all you need is the downpipe, and pair of Flowmasters, and you are good as any kit.
The website is
www.hoganperformance.com Check out part # DP5S or DP7S depending on your needs. They are very good about answering any questions that you may have.
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2007 Shadow Gray Metalic GXP
K&N Air Filter & More goodies to come.