A downpipe and a tune is a good start. A stock 2007 WRX typically dynos at about 190whp. With a tune and stock downpipe, you're looking at approx. 200whp, so not much of an improvement for just a tune. A pro-tune with a 3" downpipe will put down approx. 225whp (these numbers are debatable and depend on the dyno... however, all are borrowed from Cobb testing on their Mustang dyno). So you will see approx. 35whp increase over stock with a pro-tune and a 3" downpipe.