Mileage depends on your driving habits. But there are people out here, including myself that have seen a raise in mpg (1-2 mpg). The Air filters will not void your warranty. Unless the dealer supplies free filters,(oil/air)this will not void your warranty.
There was a court case involving this very subject. It was determined that if the dealer does not supply free filters, the dealer can not void your warranty because you used aftermarket filters. Magnuson-Moss Act.
Click
http://www.mrinkman.com/Warranty.htm