The older gens are good to about 300whp on stock air box. But, you will see small improvements once you are above stage 2. More improvements once you keep moving the hp up...but nothing too drastic. I'd bet still only a 10-20 hp total jump even when you are balls to the wall.
378awhp/373wtq Custom Dom 3 & CBRD tuned 2006 STI.
"When in doubt...flat out" - Colin McRae