I can personally vouche for Borla. I have the S-Type catback on mine and to me hands down Borla sounds the best. But.. you also get what you pay for. Borla is some of the most expensive too. GT500 sounds good but everybody and their mother has it. I wanted to be different and personally I love what I did. I still have the cars on but its plenty loud. WOT its an animal!
Here are some clips of mine
Old videos, my car has changes quite a bit
http://www.youtube.com/watch?v=4NEtUoNDVAU
http://www.youtube.com/watch?v=wm6UHO61WtI
Here's it on the dyno WOT
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Airaid CAI | Borla S-Type Catback | Amsoil 10w30 and MTG Trans. Fluid
382hp 387ft/lb | 1/4: 12.9 @ 112mph