If the K&N filter makes the bike louder, this might be a psychological thing- running it harder because it sounds cool?
The only way to find MPG accurately is to keep a running tally- I made my own spreadsheet- after several fill-ups you will be able to get a good picture.
Plus I fill up each time, to make it easy. After several tanks, which eliminates overfilling sometimes or underfilling sometimes, you can get an accurate MPG.
Personally I would not run K&N. You can clean the stock filter with Simple Green many times.
I ran K&N in my Shadow because the non-cleanable paper filters cost as much as a K&N. And I was living in NW Arizona (the dusty desert) so a reusable, oiled filter made sense (cents too)
But others have stated that K&N allows more air to flow = larger pores = more dirt in engine.