Takeaways from a Canadian after watching every team Canada game (just one rube's opinion):
Mateychuk showed his pedigree and I agree he looked visibly a cut above. Outside Celebrini he seemed by far the highest iq on team Canada. I really liked his play without the puck offensively. He made some really good reads and there were many times he was in the right spot to be dangerous or support the puck and his teammates didn't necessarily make a high IQ play, but at the next level he'd probably get rewarded for it more.
Ironically, my only gripe was his play away from the puck defensively (and his gap control off the rush a couple times). At times he looked a little unsure positionally away from the puck, including on the pk (maybe a new system or different set of expectations?).
It's very hard to project a junior tournament to playing against men, but he engaged physically at the right times (timing is important to me) and I don't worry as much about his size as I used to. He's smart about it. It won't be a strength, but I think he can disrupt plays in the NHL passably. I wouldn't want him in the NHL next year, but that is just the age old desire to let guys marinate too long rather than too little.
Dumais... The biggest thing is I liked his positioning, he seemed to be in the right spots all the time, and I think he managed the puck better than most his teammates, which I guess is a compliment. I agree with major that he was effective disrupting plays, getting in on the forcheck, getting his stick on pucks, etc. His IQ and being in the right spots helps make up for his skating, which didn't seem to hold him back.
That all being said, when you go from watching him to Brindley, the 'disruption' skills feel redundant, and Brindley definitely does it better, with more energy and tempo. If I didn't know from before, I wouldn't have seen Dumais as having any type of game breaking skills or being an elite skill guy. I am going to chalk it up to a low sample size, and I still see Dumais as a three year long lottery ticket. I don't think his impact was nearly as negative as people say, but at the same time he had a lot of opportunity he didn't seize, especially on the powerplay.