Airflow is key in producing power, especially up top. This might start a debate, but that is why we are here on the forum.
K&N filters flow more air than oem stock paper filters in cars and the like, yes. Yes they can scrub the air down to microns like a hepa filter also. But they don't flow as much air as a quality foam filter in bikes. Let the rangling begin!!
If you are going to put a K&N pod on your KX, its going to be pretty small and have a very small surface area compared to the airbox foam filter. But if you keep it super clean and oiled in specs (very lightly), it can work good.
Foam filters need to have a protective airbox and some structure to help keep them in operation. K&N's have a large gathering in applications where looks and not having to custom fabricate an airbox is more important.
Some thoughts from an insomniac
..... Sly