Most bang for the buck for a stock SVT...i'd say a y pipe of some sorts. Then, probably any exhaust upgrade (I also went custom), then I'd say freed up intake (any upgrade is better than none). That's bang for the buck IMHO.
Removing your catalytic converter, and maybe even replacing it w/ a high flow cat has the possibility to void emissions warranty...whatever that is (I'm not making that up, am I?). Also, by putting aftermarket parts on your car, Ford will not cover those parts under their warranty. But that should go without saying...
