A 1998 Impreza weighed about 2800 lbs ( the coupe and sedan less, the wagons more). Based on 100whp a GC style impreza has a p/w ratio of 28:1.
A 2002 WRX weighs about 3100lbs, with 170whp its p/w ratio is 18.24. To equal that p/w ratio the GC owner only needs 152hp at the wheels. This can be achieved with cams, intake, headerback and EM. New these parts would run about $2000.
A couple of points. First, to use chassis dyno numbers to calculate lb/hp ratios is rather silly, since we'll never know how much those "stock" numbers are affected by drivetrain losses. Why throw additional unknown variables into the calculation when we already have perfectly good HP numbers to use? We already know the crank HP numbers, since those nice people at Subaru tell us what they are.
Using 165hp and 2825# for the 2.5RS (advertised 1998 numbers) and 227hp and 3085# for the WRX (2002 sedan advertised numbers), the GC 2.5RS has ~17lb/hp, while a GD WRX has ~13.5lb/hp. Just using the correct numbers we've closed the gap from a 55% difference (28:1 vs 18:1) to a more realistic 26% difference (17:1 vs 13.5:1). Hooray!
Second, show me anybody that's making 50+hp over stock without touching the heads. To say one could go from 100whp to 152whp with just intake, exhaust, cams, and EM is extremely optimistic. I think Matt is the only one with dyno-proven results showing 50hp over stock, as he has head work done. I'm in the 40hp, maybe 45hp over stock range, again with head work done. So add a pretty healthy chunk on top of that $2000 to pay for the head work.
(As an aside, a "cat back exhaust" is everything after the cat. Hence, a "header back" would be everything after the header. I think what you really mean is a full exhaust - header, cat, midpipe, the whole shebang. Without the header, you're not gaining much of anything, as you already know.)
Anyway, to achieve the same ~13.5lb/hp of the stock WRX, one actually only needs ~208hp in that 2.5RS, which is around 40hp over stock. So, using the right numbers to do our initial calculations makes our job that much easier - we need 10 fewer HP to equal the WRX!
Some really good information summarized in this post, Rally_wgn, and I'm not trying to criticize (despite my typical smart ass tone). As an old school N/A type guy (one of the first here, I think) I just wanted to add some clarification.
'97 Legacy 2.5GT sedan (maybe a 3.0GT sedan in the next couple/few months?)