[Hammer]

Thanks for the info SVT, it really helps when advising others to know what you're working with.

As Dark said, the best bang for your buck is going to be gears. I'd like to have some Cobra owners jump in, but off the cuff I would advise going with 4:10s...

As far as the exhaust is concerned, most of your power increase would come from a mid-pipe replacement such as a Bassani X-pipe, the catback system is more for sound than anything else, as the stock setup isn't very restrictive. (In my experience, nothing much is shown from a dyno with a cat-back only swap... although I have seen much more dyno numbers on GTs than Cobras...)