If you've got a GB270, then its wrx based so will cost a fair bit more to get to 400 than an STI would unfortunately.
It has the PPP on it, so still has the big cat in the downpipe. Decat exhaust & remap would be a good starting point and then decide where to go from there. All depends on what you want to use the car for - whether you go chasing bhp or focus on handling/brakes.
The 5 spd gearbox won't last long at 400bhp either if you drive it hard, and the 4pot brakes aren't up to much if pushed.
Plenty of things you can do if you want to get your wallet out!
edit/ Nige has a very good point about insurance, at 21 thats going to be a fortune!!