I won't even pretend to guess how good someone I've never seen pitch could get. I think every prospect with very good tools has the potential to have it click and become phenominal. In Bill James' Historical Baseball Abstract he said something very interesting about Pedro Martinez.
To paraphrase: The difference between 7! and 10! is only 3 things (8, 9, and 10), but in terms of magnitude it's 700x greater. With pitchers, three little things like a little extra velocity, a little extra movement, and little better command, combined with a little extra deception (changing arm angle, speeds, etc) can have the same type of impact magnitude. Pedro didn't have the best fastball, or the most movement, or the best command, or the most deception, but when you combine everything, he's that much better than everyone else (not 700x better, but you get the idea).
This was from the 2003 edition of his book. Any of these things could happen with Kelly, well except for maybe the deception part. If they do, he could be very, very good. So, his ceiling, like that of many others, is very high.
Now, how good do I think he'll be? If he makes standard progress, he's probably a 2-3.