I have a C5 with Vararam cold air intake and borla stinger 2 catback what is the next upgrade to do. I am thing either underdrive pulley, 160 thermostat, headers, or pocket programmer. which would give the best bang for the buck?

 

Dont waste your money on a programmer. I would do lt's and good tune. That will make the biggest difference. Obviously it will be more expensive then the other options but it will be worth it.