If it's HP you're looking for a catback isn't going to do anything for you. One or two HP tops. To really free up your exhaust you'll need an off-road or hiflow h or x pipe. A catback alone is basically just a sound modification. If you want a loud throaty sound a lot of people go with Flowmaster 2 chambers. There are so many setups out there it all just boils down to personal preference as everyone has different taste. Do a search on mufflers and exhaust on this website and it should direct you to a ton of old posts on this very subject. You should be able to learn a lot from that. Good luck.
PS - I have Borla on mine and it's quite mellow but louder than stock. The addition of an H or X pipe will increase the sound even more.
Brett
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66 Tahoe Turquoise Coupe
Laser Red 96GT Ragtop
30 Years & 30 inches apart in the driveway!