My understanding is that K&Ns aren't real flash at actually filtering... If you hold one up to the light, you'll see many spots where the light comes straight through. And even the K&N blurb says that they filter better when dirty, which I assume means that they require some built up dirt to close up those holes...
I do know that my rally car picked up a few HP changing from a Uni filter pod to a K&N, but I'd never be confidant using one on an engine that I expect to operate in other people's dust.