Bear, while I agree with you that intuitevely it should make little difference, a pitcher is bound to gain some velocity when he is on the mound. Although probably more significant is the fact that, with cleats, a pitcher's planting foot does not slide like is does with sneakers which allows for the pitcher to focus his power more effeciently to the pitch. I'm also sure that having a downward motion must add some advantage, or else I would ask why is there a mound for pitchers in the first place?
That being said, I would say that the difference is negligable, and, while 3MPH is possible, so is .000000001MPH, and I by no means am qualified to say what that amount might be.