Well, that's it -- unless or until Kelly increases his FB velocity a few mph, he has no chance of being more than a fringe pitcher in the majors. We'd better hope he picks up his hitting and defensive play at SS.

This speculation is bullfeathers to a large degree IMO. The key is command and control, and movement. The great majority of professional pitchers have average command and level of movement a bit (the old normal distribution theory). Therefore, of those, the guys who can throw it a few mph faster with that level of command are of course a step ahead of the game. But only a step ahead. Jason Neighborgall had a plus plus FB and just about the worst command and control of it ever seen and he was a dismal failure as a pro. John Wasdin had a little above average FB, quite good control of it, but had virtually no movement on it and minimal command, and was a fringe ML. Greg Maddux had a slightly below average FB and otherworldly command and late movement (and the smarts to make best use of it) and fashioned a HOF career out of it.

GIVEN your capabilities in command, control, and movement, it's helpful to have a faster FB than a slower FB, though (and here is the point) you can survive (Dontrelle Willis) or thrive (Maddux) without a top-flight FB. But if you don't have at least a modicum of command and control, and/or some movement on the ball, you can throw as hard as you want and you won't do doodlysquat.

Of course, to every, even almost inviolate principle, there are the extraordinarily exceptional counterexamples. Nolan Ryan simply blew through his early command and control issues because he was almost impossible to hit (though he did become a better and more effective pitcher when his command improved later in his career.) Tim Wakefield on the other hand has no FB, command and control issues that come and go, but has astounding late movement on his knuckleball, which can make him almost equally impossible to hit when he is on and which has allowed him to approach 200 major league victories. But these are the exceptions that prove the rule.