Based on the chatter I have heard, it adds about 0% because the induction area on the hood is negative air pressure while the car is moving, meaing that the air cannot "ram" into the engine.
Even so, the cold air induction has to offer some gain, and the sound that the air rushing in makes is impressive enough that I'd be willing to spring for the money if couple of thousand bucks was just another little ol' check out of the checkbook.
