If a scrum coach can sort out his technique and conditioning guys can sort out his gait he could be a monster. All you have to do is look at his frame to understand his potential - it's not fat he's carrying like JJ used to, he's a certified beast. And he's 23 - plenty of time to sort those bits and pieces out and become a force.
I could be wrong but I highly doubt that Gussy wanted to let him go - someone suggested on the board recently that Josh was weighing up offers to leave versus what Quins could offer for him to stay, and that Gussy was trying to persuade him to stick around.