This might open a can of worms for K&N owners so I've put my flame retardant suit on, but some years ago I read a comprehensive test (no, I don't recall where I read this) comparing K&N with various other types, pleated paper like is usually used in OEM's, and oiled foam. I had been using K&N's in my vehicles and bikes untill then. The test showed that pleated paper was the best, oiled foam second, and K&N last. K&N gets their claim of more power from the fact that it lets in more air thru its porous material.
More air, more dirt. I don't use them anymore.
OTOH, I doubt any of us will keep our bikes long enough for it to make a difference, so if it makes you feel good to use K&N, then do so, as thats really all that matters.