i stil maintain that they starve the motor by reducing intake volume, the increased throttle response is the same as putting on a small carb, increased throttle response cause the air moves faster but restricts air flow at high engine speeds when the motor demands more air, less can get through.
put that thing in a glass of water, measure how much it displaces and then meaures the volume of you intake and subtract the water displacement from the intake's and you will see what i mean...
another good way to test, put it in, do plug chops and if you need a smaller main jet, because its rich, it means you getting less air.
its an expensive way "decline" not "mod" IMO