Air silencer
Air silencers are mufflers for your intake. Like mufflers, they restrict flow to a certain degree. If you remove it you can achieve a more efficient intake. Less restriction at the blower inlet plenum= more outlet volume, i.e. more boost. I have heard of 5 to 10 horsepower gains, alothough I cannot affirm, its not enough to notice from the seat of your pants. It also makes your intake a little louder, but I love the way it sounds.