He already has a VF52, which is the same as the STi turbo except for the 2 bolt outlet flange (designed to bolt straight onto the IC). Bottom end between STi and WRX are exactly the same, although the heads differ.

If I were you, I'd forget chasing 400HP at the wheels unless you're prepared to start spending huge money on upgrading your engine, ancilliaries and driveline, as well as your brakes, etc etc. Even with an STi as a base, it will still cost a fortune to do it.

I'd look at the rest of the mods you can do.. silicone intake, IC upgrade, headers, etc etc.. you'll make 300 and a packetload of torque, then you can concentrate on your suspension and some tyres, and learn how to drive the car fast. It'll be much more rewarding and less headaches!

Just remember, a 300HP at the wheel WRX is FAST.. don't go looking at what your mates do and don't have, especially if they are driving something thats not a Subaru, as the numbers always lie when it comes to Subaru's and dyno charts.