My stock GNX muffs were toast when i got the car so I had a local guy make a custom exhaust. It looks just like a factory system but I had him do it in 2.5". He used Magnaflow mufflers that DO NOT look stock, but they are a straight through design, fit well withing the limits of the GNX rear suspension, and they sound nice (mild and smooth not obnoxious).
After having this system built, the guys at GN1Performance said that their kit would work all the way back to the tailpipes and then you would have to do some customizing to get the rest done. At least this gives you an option.