This is a good source of information on engine performance. Duncan talks about taking it to 300 bhp and says an induction kit won't improve performance. Just need an upgraded panel filter.
http://www.racedynamix.co.uk/remap/subaru-remapping/typical-upgrade-path-newage-2-0-wrx/
I have a 2.0 wx blob. So slightly different car :D
I did Pipercross panel filter, WR Sport back box (it was a standard option for my car from Prodrive, I got it 2nd hand off eBay) and a Walbro. Duncan remapped it to VPower and I got 272bhp.
I didn't really do it for the extra power. I wanted slightly better economy (if possible) and a smoother delivery of power and better midrange. It really does feel like a new car. Theres no jerkiness going through the gears now.
As for economy, the Walbro seemed to dent my MPG figures - I get about 60miles less a tank now.....the remap a few months after has helped but it didn't bring it down to pre-walbro figures.