Development isn't always linear, or predictable. It's fine to have a plan, but you always have to be prepared to adjust it.
Johnson reminds me of Mittelstadt a bit too. Casey looked to be more skilled and was flashier. While his college production wasn't great, he was surrounded by mediocre players who fumbled one after another of the beautiful passes he would serve up to them...I watched almost all of his games with the Gophers. I don't know what it is that Mittelstadt is missing- it seems to be an intangible(hunger? soft? mental toughness?)- but you would hope that Johnson is not missing that quality.
I don't see anything at all wrong with Johnson right now, though. I can easily see his skating improving with a bit more strength and training to the point where he is an above average skater. Good size, excellent puck handler, good shot. Not sure about his ability to take punishment, and still play his game(one of Mittelstadt's failings). Equally comfortable passing and shooting. I can easily see him being a #2C in the NHL, or a first line wing.
I really like him. Like Beniers a bit more, but am open to change my mind on that. Would not be unhappy to get either player, if my team is picking in the top 10.