As Jim pointed out, one area that is overlooked with these "performance filters" is that they allow more dirt to enter the engine and engine oil.
Think about it: what is the filter there for in the first place? If a K and N were a great thing for performance, well, no filter at all would be better?
Actually, back in the bad old days, we used to just through a chrome cover over the carb and let the thing wail!
No wonder I had all those engine "problems."
See the Motor Oil Bible for more on the importance of a quality air filter to engine life.
www.themotoroilsite.com