Stock exhaust is the factory X pipe with chambered mufflers and an angle cut tip.
No brand other than OEM ford.
Best place to search for exhaust sound and what suits you best is to go to youtube and search Gt500 exhaust. Plenty of people setup different videos, then you can see what fits you the best.
2008 Mustang Shelby GT500 (Best time: 11.95 sec @ 119.2 mph)
1 of 80 Performance White/Ebony Stripe combo made in 2008
2007 Mustang GT/BOSS281 (Best time: 13.6 sec @ 107 mph)
Satin Silver/Black Stripe combo