IMHO any axle back free-flow muffler will improve "breathing" of your engine over the stock GT exhaust which has baffles. I like Magnaflow products and their performance exhaust system I put on my GT "does the job" for me. It is completely free-flow, without any baffles so the engine definitely breathes better now. It has a low, mellow, sporty sound which is not too loud.
If sound is of primary importance, you may want to go to a "PT Meet" to see/hear various exhaust systems in order to determine which one you like best--a "cool" sound for one person maybe "noise" to another.
Good luck.