Not to be negative, I've owned 25 Subaru's, and that is what you got under the hood, is a Subaru Boxer 4 N/A. I've owned WRX's, Legacy GT's, Forester XT's and several Impreza RS's , coupes and , wagons. Your N/A 2.0-2.5 boxer engines are not easily modified, and you really don't get to much performance for the money spent. Cobb used to make a Accessport for N/A engines, but they got out of the business , there wasn't enough benefit to pay over $500 to get very LITTLE gain in performance, people just quit buying them, Cobb just deals with turbo model cars. Your turbo 2.0 and 2.5 engines are totally different, so parts are not easily swapped. The only way to gain significant power out of the boxer engine is to turbo, or supercharge it. You might be able to gain 10-20 hp with items mentioned like exhaust, air intake system etc. but you will not see the same amount of gains as you would in other models, it seems Subaru doesn't leave alot on the table to make gains from ad on's. At least in my experience. There are not a great deal of parts available either, and with the limited of number of these cars to be sold, Subaru saying no more than 2500-3000 for them per year, Toyota/ Scion???? I really don't think the aftermarket is going to be very quick in getting out performance parts, since Subaru is already stating they are coming out with a turbo model within a year.
If I had a crystal ball , I would say, unless you plan on rebuilding the engine with performance parts, costing $$$$$$$, or putting a turbo on the car, those will come , may take a while !!!!! There isn't going to be alot to be had in performance parts, and when available not much power to be gained. That has been the history of Subaru's boxer N/A engines !!!!!! SAD BUT TRUE !!!! Things might change, but don't look for it, Scion folks, might hope that Toyota changes their engine offered in the FRS.
Another thing, although the boxer engine is good proven engine, I believe that there were better choices to be made for this application. I'm a loyal Subaru fan, I've also owned several Toyota's, but they should have put a better engine, with performance ad-on capabilities, in this car, just my opinion, everyone has got one !!!!!