Nothing in the long run. It would make the intake air slightly cooler and the engine marginally more responsive. And you might have to clean/replace it. Air filters like that do bugger-all unless the OE one was really crap
they'll only really start having a positive effect when combined with other mods.... esp. if you do FI (turbo, or supercharger)
both of the guys are right up there, i mean i wouldn't just get a filter i'd get a whole intake kit. I know that the K&N cold air's are exceptionally good really worth the price, mine kicks ass. i noted about a 1-2 mpg increase in the first tank on a v8. Throttle response also slightly increased and i had a little more high end torque. i mean other then that it didnt do a whole lot towards power or torque.
just makes the car more efficient..remember to reset the ecu on any upgrade.. ~Earnie~