What effect would a performance air filter make on my car?

You won't feel any surge of power. Maybe 1 or 2 HP. The performance air filters let more air flow by increasing the size of the particles they filter. More air flows and so does more dirt. You won't see much of a power increase because you still have the same intake and exhaust size. The same volume of air will flow unless your old filter was clogged.

no increase in power that you can feel by any means, it sounds a little better but unless you are going to do exhaust modifications you wont really get the full Subaru sound potential, you want something that sounds good put on a cold air intake, a Flowmaster 40 series muffler, you'll be happy with that, if you want to go further from there get a full catback exhaust and a Borla header to go with however this option is substantially more expensive, to see any decent power increase is not gonna happen without some major money tossed down, it'd be cheaper and more reliable to get an STI, there's a lot more performance parts for these vehicles too, most of the mods you will find for your car are going to be for handling/suspension.

I agree with both those guys. Don't pay some obscene price for a so called 'performance' filter and expect to burn rubber just because of it

Performance filters don't automatically let in more dirt. They're designed different, and they still filter just fine. That said, you won't notice anything as far as a power gain but you may enjoy the new sound. A potential benefit though is that the filters are often washable and reusable. Depending on the environment you drive your car in and how often you change the filter, it may be worth the initial upfront cost because you won't ever need to buy a replacement. They can typically be had for <$40, which isn't really that much money in the grand scheme of things. Get one if you want one.