K&N filters are great... if you hate your engine.
Intake restrictions limit peak power in modern MAF/MAP sensored engines, similar to having the throttle butterfly in a partial restriction. But they don't affect your overall mileage. Your mechanic brother sounds like he learned on carbureted engines.
EDIT: Also, for partial loads like that, ecomodders generally recommend the reverse of a CAI, which is a warm air intake.
https://en.wikipedia.org/wiki/Warm_air_intake