If you enter your car in an autocross STOCK class the warranty should not be voided. An autocross is not considered racing but is considered a low speed timed event. Even Solo I where you are on a race track by yourself is still considered a "low speed" event, not a race. If you strip the interior, take apart the engine and modify the hell out of it and enter a Modified class event I would say then you are on you own. If you enter an SCCA T1 race where you can swap paint then that is racing. Remember "Rubbing is racing" (Days of Thunder).
Under SCCA rules I may replace my stock wheels,tires and shocks for STOCK events. If I blew a motor. I would definately change back to the stock wheels, tires and shocks and remove any stickers before I took it into the dealer for repair under warranty.
I would also shrug my shoulders and say "I dunno what happened, I was just driving along and BANG it stopped." Similar to the answer I always get from my 14 year old when something breaks around the house, "I dunno, I didn't do it"
AND IF THEY SAY "HEY, I HEAR YOU AUTOCROSS THAT THING." I would say "What's autoWHAT?!!!"
I am not at my office with scanner so I will not type the whole thing but on page 8 of the Warranty and Owners Assistance Information is says
What is not covered:
Misuse of the vechicle such as driving over curbs (DOH) overloading, racing, or other competitions. Proper vechicle use is discussed in the Owners Manual.
Alteration or modification to the vechicle including the body, chassis or components after final assembly by GM.