I think a lot of guys have seen him a lot since Avangard is a big market team and played many many games in the PO. But since the game in the NA is so much different I don't think you can get a definite answer here.
Very good skater for his size, no problem there. Offensively he's good, he has a good shot, is a good passer and sound puck mover. I think these qualities of his are at the level of a top-4 NHL D-man already. It remains to be seen if he can survive in his own zone though because the game is much different in NA. If he learns to use his huge frame I can see him becoming a legit top-4 D but it will take some time. He can definitely be used on the power play but if EDM will use umbrella formation on PP he's not the best option because well, you don't see too many 6'4" D playing on it.
Long story short, he has all the tools of top-4 D but his ability to adapt to NA game will determine his success. But he will have to change his style, his mentality, play more games than ever before and take a lot more banging than ever before.. I would be surprised if he made an instant impact, patience will be required.