RPD: I'm basing it off of the assumption that the ball slows down 1 MPH for each 10 feet it travels, and it travels 127 feet to second base.
A 70 MPH inital velocity then has an average velocity of 63.65 MPH, which takes 1.36 seconds to get to second base.
A 75 MPH initial velocity has average velocity of 68.65 MPH, which takes 1.26 seconds to travel to second.
I think you are failing to take into account deceleration of the ball in flight.
Fungo: I agree that a radar gun would tell us if there was an arm strength issue, but I'll betcha I could tell right away without a gun if the kid was throwing down at 56 MPH. The ball would either bounce three times or would have to be a rainbow.
I don't think anyone is trying to take arm strength out of the equation here. That particular item is very important to my son, and totally agree with you that mechanics are far easier to teach than arm strength, meaning that for play at the next level, arm strength is arguably more important.
But that wasn't the question as I understood it. If the question is how to most quickly reduce a given catcher's pop time from 2.4, it is pretty obvious to me that the answer lies in release time.
That is, unless I see that rainbow!