On a bone stock NA engine, the only thing you'll gain with a special air filter are louder intake sounds. Modified & turbocharged engines are a whole other animal then. If you are concerned about fuel mileage, it'll be cheaper & more effective to just put a hockey puck under the gas pedal ;-)

The whole idea behind engine upgrades in a nutshell is to be able to burn more fuel and therefore create more power. This is very easy to accomplish on a turbo engine (increased boost pressure shoves more air/fuel mixture in the engine) or older, large displacement engines that were hampered by restrictive carburetors & cylinder heads, e.g. Mopar 225 slant six with single barrel Holley vs. the same thing with HyperPak intake & 4bbl carb.

As Jeff pointed out, you're the lucky owner of a NA "factory hot-rod". In plain english, better get your checkbook out as there are no cheap solutions like there are for turbos ( such as Saab APC computer from a junkyard or manual boost controller valve).