The 'brand name' exhausts you see spoken of around here as well as customs like Flowmaster and Magnaflow all deliver about the same horsepower. Pick yours based on looks and sound. For the best NA power, get a full MRT setup or the Brullen headers with a good cat back.

 

The problem with add-ons (like headers, free flow intake, lightweight pulley kit, and cat-back exhaust) is that they cost a lot of money, but they don't increase power proportionally. Typically it is only realistic to expect that these bolt-ons when all added up will net you like 20 horsepower if you're lucky. The only REAL way to increase NA power is to change cams, increase compression, and have some headwork done (at the least). My advice, for what little it is worth, is to hold off your immediate want for power and forget all the little add-ons, as they are definitely not worth the money you'll spend to get them. If you concentrate on handling enhancements (like larger anti-sway bars, adjustable KYB shocks/struts, higher rate springs, and good tires/rims) you'll be much happier with the results you get for the dough you spend...the car will actually SEEM like it has more power. The NA Impreza, while not necessarily fast, still, even at stock power levels, can be made significantly better by upgrading the chassis/suspension first. Even at stock power levels the stock chassis can easily be pushed beyond its limits, and plus if you upgrade the suspension, wheels, and tires you will have created a much more finely suited home for any eventual forced induction or high-output NA power mods you have in store for later.