I think the change in sheer number of games, not only as a whole, but within a stretch of 4-5 days is the biggest challenge. I would not be surprised to see him in GR for at least the next 13 months - if, for nothing more, to allow him the chance to build his endurance. Not just skating, but in the weight room, in practice, and the physicality of the game. That's gotta take some getting used to before someone is ready.
Do I think he could come up and be in the 80th percentile for three or four games? Absolutely. Do I think he'd regress to the 20th percentile after 60 games? Absolutely.
I think you leave him in GR just so he can get a feel for how to battle with a balance. You can't go 150% all the time every time.