OP, this article is a nice read, shows before and after results of each bolt on item, cai, catback, and pulley. I have done the cai, and catback with no tune. My results are not much over stock. I plan on a pulley with tune this winter as well as upgrades to the suspension, I have bad wheel hop. The results in this article seems to be consistent with what other guys are getting. '03-'04 Mustang Cobra Performance Test - 5.0 Mustang & Super Fords Magazine