Errmmm... I also experienced troubles when I took the filter out of the stock airbox... I ran consistent 14's.... Thought "more air = better" take the filter out and lost about half a second!

I don't know if that is due to the stock airbox designs maybe? Could be something to do with the resistance of the filter helping to smooth the air out.... I think Injuneer nailed it with the turbulence killing the ET.