I am not the guy to be asking about exhaust sort of stuff, but we put a K&N filter in our X and immediately noticed a difference in performance, and between 1-3 mpg increase in the past 2 months. I think that the replacement filter is a better deal than replacing the airbox with one of those pop charger kits. The box does help keep water out of the intake to a certain extent, and would rather leave that piece of my vehicle in there for cheap insurance.

Do a search on K&N and I'm sure you'll get plenty of information.