The thing with Mestemaker is that his stats were exceptionally impressive, but it's about when those stats were accumulated that will determine if he's made that "next step" or not. For example, 6 of his 9 interceptions came against the two ranked teams they played last year. For sacks, 7 of his 18 came in those same two games. He'll continue to physically improve, but that growth (IE: his ability to make "all the throws") will be more limited because he is already so capable. Where we'll hopefully really see him improve is mentally. How much more he's really wrapped his head around HCEM's offense, knowing where players are supposed to be, how much time he should have in the pocket before needing to get rid of the ball, etc. If he can take a step forward on the mental side, our offense could be really, really good.
I did read that HCEM wants to prioritize HS recruits at the QB position, which is music to my ears. Outside of elite players, I think QB is one of those positions where you don't get nearly worth what you invest in bringing in transfer QBs, especially in that first year. QBs really need time in an offense and HCEM's approach gives a QB that time. I may be old school in my thinking, but I'd much rather have a QB who's super intelligent but raw as a prospect than a QB who's polished mechanically and/or has a powerful arm, but doesn't have much between the ears.