K&N filters are rock catchers not filters, they flow very well at the expense of filtering as you'd imagine you can't really have one without the other.
Look inside your intake hoses mine had a nice coat of dust which magically disappeared after fitting a proper Bosch paper filter..
The Oil on K&N almost always finds it way on to the MAF too which causes no end of issues.
I didn't notice any drop in performance or MPG without it..
15mpg is about right if it is crawling around town or doing short journeys with steep hills etc
Mine will usually do 18 around town and the best was 26mpg on the M4 at 2600rpm which is around 70mph
done with a brim to brim, no trip comp nonsense, however mine is accurate to a few tenths so it can usually be trusted..
Yep, F1 engines don't have to last that long!