i have this smell as well. it usually comes from the back of the car when my tach-0-meter goes to the red part. its smells like rubber but im not sure. last time i had a buddy follow me to track down the smell and where it was coming from, but when i tried to replicate it, his LS1 couldnt keep up, and i think hes still at the light to this day, coughing from the smell.
anyways, you could just replace the belt and find out, theyre like 30 bucks i think. if theyre glazed then theyll just slip under load.
if you were manual id ask how your clutch felt, but nevermind on that.