Brian,
the really sad part about the stock GTRA mufflers is that not only are they loud, but they are heavy and also kill horsepower, particularly on modified engines. As we found on the dyno with my car after the engine was modified, it picked up 20 ftlbs of torque and 12 hp when we cut the mufflers off and re-ran the car.
I have had great results with Spintech mufflers and Superflow mufflers. The Superflows are probably a little quieter but depends on the series that you get. The Spintechs made excellent power on my last race car, 308 cube motor that was 466rwhp on a Dynojet dyno. I would not hesitate to use either of these brands and have used a 4" in/out Superflow for my new Panoz system. Car sounds great but is still reasonably quiet.