I could see him getting his cup of tea, aka 9 games and then sent back down. He's in the WHL, so it's not a big deal. I imagine they would do the same with Johnson if they could. But him playing at Michigan prevents that.
I don't think it's crazy that Sillinger could earn a 9 game tryout.
What I've gathered from watching more video on him is that he definitely got away with things at the USHL level, and he didn't use his teammates as much as he should have because he was just too much better than them. WHL might be a bit better level of difficulty for him, I'm not sure if there's much of a difference there. All of his basic skills are strong (stronger than PLD in his draft year), and he's already 200+ lbs and very strong for an 18 year old. I normally would rule it out, out of hand, but I look at the player and I don't see him missing anything obvious that would prevent him from keeping up. His team play and defensive play were inconsistent game to game but he had games where he was really dominant and I got the sense that he was just playing down to his team. The size, strength, and sense are all good enough. The skating is funky looking and can be improved but he is quick enough. He would have one of the best shots on the team from game 1.
Johnson though, not a chance in the world would I give him a tryout. We took a big swing on a guy that might develop into a superstar. We know he's not that good right now. I believe I picked this up from Scouch - KJ only had primary points on 37% of Michigan's goals when he was on the ice. That suggests he was mostly lucky to pick up a point per game in his freshman season. I expect big growth from there but I don't think his current form is very advanced.