theres plenty you can do to a mustang for performance! first i would get a K&N air filter and rip out the air silencer. then get headers, h pipe and a less restrictive exhaust. next up grade the gearing to 3:73's or 4:10's for more pull. you can always bolt on a supercharger for a 40-60% power increase. you might want to check out
Mustang Works there are a lot of people there that know a ton about mustangs and would be glad to help you out!