i recently had the airbox out and noticed that it has a 'horn' at its intake, where outside air comes in. to me it seems that the shape of the horn unnecessarily restricts inlet opening, and what's worse, it essentially prevents the intake air from getting to most of the air filter surface. the result is that only about 1/4 of the filter is flowing vast majority of the air. this is supported by the fact that one corner of the filter gets much dirtier than the rest of it.
i found that the horn is held in place by a single philips screw and can be removed (and reinstalled) in literally less than a minute. removing the horn really opens up the intake and lets air get to much more of the filter surface. because the airbox intake is so close to the 'cold air' duct, it looks like it should act as its own 'heat shield', taking vast majority of the air from the duct, and none of the hot radiator air.
after removing the airbox intake horn, i seem to have improved throttle response and acceleration - my butt dyno would guess about 5hp. if anyone is taking a stock-intake ///M to a dyno anytime soon, please check this mod - it only takes 45-60 seconds! full stock filtration is retained, so i can see no downsides - and if it actually picks up some horsepower, it would be an awesome bargain!
please post here or email me if you have a chance to try it or want more details.