If you're happy with the stock power and exhaust sound, your wallet will thank you. What people like about exhaust sound is very subjective; you can only really go by your own ear and what sounds good to you. Louder doesn't necessarily make it better.
The stock exhaust system is a little restrictive as there is 30-40 more hp to be had with a header system, cai and a tune. I would listen to other GTOs at car shows (if you can find any) or LS2 Camaros and listen to what they sound like and go from there. Enjoy your car!
1-7/8" ARH LTs, 3" Magnaflow exh.,Spec twin disc clutch, 402 stroker , Fast 102 w/Nick Williams TB, 1000 hp shaft & axles, cam, fuel & suspension upgrades. 500 rwhp, 484 rwtq