Its a grey area.
#1 talk to your dealer and be open. get a feel for them and how they take it. If they immediatly say "xxx will void your warranty" find another servicing dealer.
#2 find a shop that has ties to a dealer. they can work together and get most things covered.
#3 mods only void the warranty for things that are in direct relation to the mod. i.e.
A. If you add long tube headers and your radio stops working... youre fine.
B. If you add 4.10 gears and slicks and your axel snaps... ehhh you may have an issue UNLESS you have a good realtionship with your dealer b/c GM reps would not get involved with this repair. The dealership could replace the half shaft, turn it in and be done.
C. If you tune your car and its a poor tune and it runs lean and burns up the engine... you are SCREWED b/c this WILL invlove GM reps. No matter how good your relationship with your dealer is, when the execs come down (they will to see what happened to the engine) and they find the tune (they will, you cant just put it back to stock, thay can see this too) they will not cover it.
If you are wanting to mod and are really afraid to lose the warranty... catback, intake, any aesthetic mods... youre fine. Avoid headers, removing cats, tunes, anything electrical (sorry AAC) like lights and aftermarket wiring... replacment headlights and tail lights would probably be OK if they are plug and play but avoid anything you have to tap wires.
All that said, I toased my warranty at 2000 miles, I am at 14,000 now and the car is perfect! Only warranty item I ever had was the
pealing paint on the rear bumper but that was fixed no questions asked.