I run a K&N filter. Playing in the desert with the silt and sand, I rather have an oil treated filter. Just my preference. I've never had an issue with it fouling the Mass Air meter. As jw0747 said, it's not soaked in oil. As for it being in an air box, I don't really see the need, other than it acting as a pre cleaner. Loss of power from hot air into the intake? Under an extreme load like towing, I could see how it could make a difference, but for everyday cruising and trail use, my motor just doesn't demand that kind of power, that the colder air difference would make. The air deflector is just cosmetic. The Mass Air only faults from the fan wash when the filter is not in place.
Mark