I've read about some parts giving you increased horsepower and still either not affecting the MPG or even increasing it.. Can anyone tell me which parts these would be i.e. new exhaust, cold air intake..and what brands are the best. I have a 2007 S. Thanks.

there are some great threads on here that will answer your questions. check the "mods" section.

keep it real homey.

I have a great deal of experience modding ls1 engines. I know that headers will diffenitly increas gas milage. Gears were the only mod I did that decreased gas milage. Even the cam kept the gas milage the same. Basically most performance mods increase the efficiency of the engine. Your engine is like a big air pump. The more air in and out the more horse power. HP mods lower gas milage when you can't keep your foot out of the gas because it's so fun. :)