For some reason, none of the desert racing teams I've seen use K&N air filters. In fact, in all of the Team Green KX 500s I've seen, they have the stock air filter, unless the rider was sponsored by another filter company. My Team Green bike had the stock filter on it when I bought it. Desert races are very dusty, and I've never had a problem running the oem filter, of course being properly cleaned and oiled.
I have seen V8s run on the dynomometer actually lose power from going from the stock paper element filter to the million mile K&N. They also can play havoc on MAF sensors and other sensitive equipment, so I have always stayed away from them.