I have/had a K&N in my stock box.

You can either get either one of two K&N filters......The direct drop in (no modification) is part E2427 and the generic size filter which requires some modification (cutting off the top of your old filter and putting it on the new one) is RU2820. The best price on the direct drop in filter is about $65 and the best price on the other one is about $35. I didn't want to mess around so I just got the direct drop in. It seemed fine.........no real noticable change in anything. I was curious about what it would to to gas milage (supposed to increase a little bit). I only ran it completely stock for about a week though. Then, I took off the stock resonator and made a DIY Icebox intake. So now, it works great and I love it! haha But I think a lot of this has to do with the intake not simply the box.