If you are looking for a scavenging effect.... I had a set of Supertrapps on my GT when it was modded and they are for sale. They scavenge exhaust very well and actually pop and crack a little when you let off of the gas at highway speeds. The GT is demoded now and the GTP had a Borla when I bought it. The Supertrapps were $300 new through Summit Racing and I was looking to get $150 for the pair. They are stainless steel and the internals are completely rebuildable.
I agree with "dayday" about the 400$ thing. For 400 bones you can have a custom cat-back made, and then some. If you're looking for that sweet sound that you dont quite get with stock mufflers (which DO actually flow pretty well), i'd go BORLA or FLOWMASTER. I've got Borlas on mine with a 2.5" custom exh, headers and HF cat and it's right where I want it. The fist time I frigged with my exhaust, I left the resonator in for fear of a LOUD horrible sound. I was surprised how tame the exhaust was with the Borlas (minus the headers and cat etc). So immediately I hauled out the resonator and it made it a little more raspier which is very nice. Sounds like Dayday would give you a deal on those Borlas, and they are an excellent muffler. Flowmaster sounds great as well, however i've never heard them on the GPs, just on GT Grand-Am's with the 3.4. Very nice though. Good luck finding that sound you're looking for.
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