would a KN hi-per air filter helps with gas mileage and power?

I heard good things about them.

In short, no & yes - some have felt that they have improved power, in which at that point you tend to use more or the same amount of gas, depending what kind of foot you have.

Personally though, after learning what I have about air filters, the regular paper filter is the best thing you can do for your engine, it blocks out and filters any dirt particules, whereas filter's such as the cones, K & N, etc allow a lot of dirt to pass through and into the bearings - any gain you may see in hp, gets lost in overall engine health, personally.