I'm surprised... that for car people no one has mentioned K&N. Hands down, best intakes on the market. Not only proven on the dyno, but also proven to be the best "filter" on the aftermarket. Ask anyone who has run them, imports or muscle.
Sure allowing more air in makes more power, but allowing more dirt in with that air is slow death. Injen and AEM are two of the worst when it comes to actually "filtering" the air that they allow into the engine, HKS is bad too (Apex'i and Blitz are not far behind, not that they make intakes for MS3s anyway). Granted the cleanest filter, and most restrictive, will be the stock paper filter. All things considered, I'll be spending the extra $ on a K&N SRI, and removing whatever brand CAI the previous owner installed.