I have an AFE Stage 2 filter system...what a difference. what i like about afe is the optional higher filtration filter you can get and i was told, by a AFE rep, that you only loose about 50 cfm which is somewhat noticeable but still much higher than a stock filter system. Prefilters are nice but they mainly just keep water out of your intake. Ive heard bad things about k&Ns especially here in montana...marc can vouch for that. One thing that is esspecially cool about the afe, and im sure the other ones do it, is that when you get on the throttle then snap release it you can here your turbo let off and its quite loud. That is somewhat bad for the turbo but i haven't heard a turbo bark yet so i don't know.