When I had my Viper GTS, the only guys who ever challegened me were Crotch Rockets, and a few Mustangs.
Crotch Rockets being faster was a given. I'd just smile and wave, and shake my head, and mouth "No thanks".
With the Mustang drivers, I figured if they were crazy enough to want to race a Viper, then they were probably crazy enough to spend the money needed to make their Fords faster than a Viper. So I'd just smile at them (while they kept reving their engine) and otherwise pretty much ignored them. I figured I had nothing to gain (by any stoplight encounter) and everything to lose.
With my Crossfire, I installed the hood scoop, rebadged my car as an AMX, and added SST emblems to each fender (back in August of 2007) and nobody has challenged me since. Probably because they don't have a clue as to what I'm packin, or have been paticularly anxious to find out because they might be really embarrassed if they do.
Cheapest "Performance" mod I ever made....well at least the "emblems" were.