Filters and intakes come up and I haven't written a big ol' condescending effortpost in a while so buckle in and lets take a look at some real info and not just some second hand ricer bullshit which we would normally see, aka 15 posts of MY KN IS AWESOME GET ONE blah blah blah shut up.
So the first thing I go to is miata guys. They're usually engineers who go to the deepest details. Two seconds of googling dumps this thread:
http://forum.miata.net/vb/showthread.php?t=420216
Not a lot of concrete evidence or links like I'd expect, so I dropped miata from my search string.
Ah bob is the oil guy
http://www.bobistheoilguy.com/airfilter/airtest1.htm
Some interesting stuff, but subaru owners are stupid, poor, and have the attention span of goldfish, I doubt any of them got through the first paragraph.
On to the next one:
http://www.nicoclub.com/archives/kn-vs-oem-filter.html
This one has graphs breaking up the words, so I think some people might be able to stomach this one. Basically KN lets a lot of flow through, but more dirt.
For all the stupid ADD people, just watch this and realize LOVING with your intake is dumb:
Sure there are times where you will get more power with an intake change, but you'd have to have a non-stock turbo and a lot of other mods for the intake to be a restriction. Run stock until your tuner says not to.
So end results?
You cannot let more air in without flowing more dirt.
Final thoughts
On an NA setup, there is literally no gains, sure you can throw an intake, exhaust, etc on, but you could just shut up and learn to drive better for free. On a turbo setup, there are better mods you can do than throw on ricky ricer ebay intakes. K&N is raisin and known to let more debris through. Yes blah blah blah you can run them and have no issues plenty of people get along just fine every day.
I have personally seen a K&N panel filter fail at a rallycross, folding in half and getting sucked into the intake, letting dirt by, and killed the turbo after we took it all apart and cleaned everything it still failed on the ride home.
I personally run 9 dollar wix panels from rock auto (PN: WIX Part # 42728 or CA9113) in my stock intake, on both my race car (that I actually LOVING race) and my DD. If I had a stock 302 mustang or an sti with an aftermarket turbo or something with more displacement/power, sure I might look into intake options, but its a stupid 4 cylinder sewing machine. It makes 110whp. Your money is better spent on tires than ricer crap.
Stop buying into marketing BS.