On the subject of any gains, there is a very small mileage improvement. But, you won't be able to tell unless you are really paying attention. It is maybe 1/2 mile per gallon. With a gain this small it is difficult to tell if the gain is from the air box or just more favorable environmental conditions. I measured several tanks of fuel both before and after. About 1/2 mpg is all I mustered.
The gains would be more significant if you also had banks Derringer or another tuning device installed i believe.