I helped a friend put a supercharger on his Toyota T-100. When we pulled his intake manifold off the inside was coated with a very fine layer of grit. It was almost like super fine silica sand. Needless to say he was using a K&N filter. It was the stock replacement flat panel type. He is very meticulous with what he does to his truck and was shocked to see the grit in his intake. He now uses the Toyota filters. After seeing that I pulled the K&N out of my Rover. I could hold mine up to a light see small holes in it. Maybe thats why if flows more air. I've heard other people say they were designed for racing and airflow was more important than engine life because the engines were rebuilt after a season of racing anyway. Personally a few HP vs. engine longevity, I'd rather have an engine that will last longer.
Check this out.
http://mkiv.com/techarticles/filters_test/2/