Air filters like K&N do nothing, other than (in some cases) reduce the level of protection.
That is unless you have race bike and you are consistently at very high revs.
Best thing you can do for 650 for mpg is take care of your bike.
Make sure brakes aren't binding, chain is lubed, FI is working correctly, Throttle bodies are sync'd (though I've read that's not so much of an issue from '07 onwards), Cruising speed is kept to or below 70 mph and most importantly... ride efficiently (carry speed, don't over brake, don't over accelerate, don't cruise fast).
I've noticed mine has a power pick up at 6,000 rpm (powerband) and that if you ride at 80-90 you are cruising in this zone. Doing this home from Folkstone a couple of weeks ago I got the 1st flashing fuel level warning at 190 miles, normally doesn't start until about 230-240.