I just purchased a new 2006 GT coupe. I want to know what mods I can perform and keep the warranty. I want to do a cat-back exhaust, underdrive pulleys, cold air induction and re-tune.

 

You can get by with the CAI and exhaust, but there is a debate over the tune. From what I've heard a tune will void your warranty because you are changing the computer settings. There is a flash date that is recorded on your car's computer so they know when the car was last flashed with a tune. As far as the pulley's go, I wanted to do the same thing until I found out that you will probably have to purchase a larger alternator as well which will definitely void the warranty. The pulley's put too much of a load on your electical system.
There are quite a few threads concerning your warranty. Keep diggin'! I think I am going to bite the bullet and do it all!