Keep the stock filter for best MPG. K&N filters may benefit high performance enthusiasts, but not any real benefit if max MPG is the goal.
Surprisingly a dirty filter has no affect on fuel economy with EFI engines, as the computer will adjust A/F mixture accordingly. It does make a difference on carbureted engines though.
http://www.fueleconomy.gov/feg/pdfs/...02_26_2009.pdf