You have a goal so that's a good start. It's a lofty goal, but a goal none the less. With that goal in mind I would suggest the first thing you should look into who is going to tune the car, they'll give you a path to take (turbo choice, injector size, EBCS, etc...). At 400 WHP, you need to look into upgrading brakes and suspension, I would even consider doing that first as '08s tend to be the "softest" WRX's produced. That's where I would start if I were in your position.
On a side note...COBB OTS maps don't support Injen intakes so your first step may be to replace that with a stock airbox. That's assuming you're using COBB maps, of course