I would not change it at all, It will hurt performance in a bad way. I ran the air box on mine right upto 400bhp before having to replace it due to Front mount inter cooler. If I could I will still run it!
You would be better off saving and changing the backbox/exhaust system fully. you can get away without a re-map changing just the backbox, changing the downpipe/ cats etc will require a re-map. getting an STI up-pipe to replace the existing WRX one is also a step in the right direction
