The issue is we have seen them and we understand what they do. Will it improve response? Sure...just as putting your foot further into the pedal will improve response...because that's what it's doing.
Now you may like the feel of having more throttle for less pedal movement and, if you do, then this works for you. Just realize that you get to full throttle quicker and past a certain point in the gas pedal travel the pedal isn't doing anything since the throttle body is already at 100% open before the pedal is.
For the average driver this affect may be fine. For those of us who push the car, we know our throttle and want the full range of it's motion. If we want more throttle response, we'll put a bigger throttle body on the car.
I'm going to contradict you a little bit here and say that it doesn't actually improve throttle response at all, it only "improves" the perception of throttle response. The driver may think it is better because X pedal movement used to result in Y throttle, and now the same X results in 1.3 Y, but the result would be the same if you applied 1.3 X pedal movement. So nothing is actually "better". Well, it might be better by 0.035 seconds to WOT, but that sure isn't worth $250.
Sky NA 2007 Midnight Blue
Sky RL 2008 Carbon Flash - Naked