I have had the drilled airbox for about a year, which yielded a slight improvement in MPG (about 1/2 MPG increase), somewhat noticeable increase in performance (acceleration), and a definite increase in engine noise at full throttle.
I added a K&N filter to the airbox last month, which, so far, has yielded another 1/2 MPG, but no noticeable performance increase or noise. But it has only been a month...we'll see how it compares in a few months.