actually as far as the K&N goes..first check to see if your tabs are broken..
that hold the lid down...baby those . that is the biggest dust culprit..
but the K&N is not the best filter..some have had good luck with them , others haven't.
the ford ais is the cheapest bought sytem...look at powerstroke shop.com for them.
the DIY tymar ...aka napa, fleetguard etc.. is the cheapest...picture a big shop vac looking filter laying next to your battery. performance wise afe2 is the king..king of price as well. personally I like the s&B ..or diy.
as far as milage goes don't believe the overhead console it is a guess at best.
you have to reset your trip odometer at fill up..run a lil then fill up again and do the math. if you changed tires etc..you will throw that off as well.
but if all is good have a look for black soot on the firewall behind the turbo...
not on the front pipes near the boots (that just means your air filter has been neglected)
but on the back pipes ..that idicates a turbo boost leak..robbing power and milage.
and above all do not ask the dealer to fix it less it is under waranty, else you will get a free ride. like a $3 o-ring set I got charged 250 bucks for a 30 min job for. always ask how much before you say sure go ahead and change them.