They give a little bit more power. Nothing to shout about though, all it is doing really is letting your engine breathe a bit more easily. I noticed no difference in sound when i fitted a piper x panel filter in mine.
I have since fitted an apiro 'aera' open filter and combined with my x-power exhaust the sound is great. Just ask the guys that where at cobham. My zed ROARS 8)
K&N panel filter made no more noise because the filter box is designed to reduce the noise. Power gained on my 25 gti was 4bph went from 142 to 146bhp. To get the noise with the panel filter you have to mod the box. Best i found was a green filter induction kit which I borrowed of a friend better than K&N. Sounds so nice and keeps the power up.
i have a k/n pannel filter in my box on my zed and it does help the engine breath a lot more and give me more range in 2nd up words on...a lot better than the origanal one..but im looking at getting a piper cross induction kit possibly like keiths(the hooded claw)has on his diesel...bloody hell does that car shift