Hoping you guys can give me some help, in the U.S. you seem to do more mods than the U.K. so I decided to ask some expert overseas advice.

I had a stock S2k for 6 months and I've started getting the bug for mods.
Recently I've just added a K/N Typhoon Air Filter Kit which was about

 

You should do a search on exhaust systems that give the most gains.

After that, maybe do a search on VAFC's and after installing some initial bolt-ons, might want to go get the VAFC tuned to compensate for the bolt ons you have. Some have been getting gains up to 15 hp to the wheels.

 

well, headers might actully give you more power then exhaust, and its not much more then exhaust, try mugen or spoon.

Sadly, if you just want pure power, this car wont go anywhere untill you spend some mega bucks.