You're warranty will not be voided outright from simple mods, but the dealer may void parts that may be affected by your mods. If you put 20's on, and wear out or break a shock, for instance, they may not honor it. If you put a supercharger on and blow a piston, or the transmission or rear end, they're not going to honor it. But if an auto-leveling headlight breaks ($1200/each!), it should have no impact, if you've put performance exhaust on, etc.
My car has Eibachs, Borla dual exhaust, cold air intake, 18" wheels, FE4 shocks and sway bars, 14" 4 piston, Brembo disk brakes all around, a retrofitted Nav, and V bodywork. My dealer has honored all warranty work until I hit my 100,000 miles. I had headlights replaced, leaky transmission seals repaired, ignition coils replaced, and leaky AC housing replaced. All, no questions asked, with a free brand new CTS loaner, each time!