the problem with the wjc's is that you are talking about 5, 6 maybe 7 games and while some are against the best of the best, usually a few of those games are against very weak countries/talent pools.
Look at Puljujarvi, he killed the WJC's at 17, MVP and had more assists then everyone but his linemates had in points. Collberg was over a ppg at 17, I believe he's around top 10 all time in Sweden for points at the WJC's. So guys just get hot at the right time, some just get in a good spot in terms of linemates vs what they are facing. It shouldn't be taken as too good or too bad, just what it is a very small sample size against the top U-20 players outside of the ones that NHL teams refused to loan out.
I don't see it, if you put Romanov in the OHL at 17 I would be very surprised if he ended up OHL defensemen of the Year with 57 pts in 67 games. Sergachev may be dumb but he's got some very impressive offensive skills, now maybe he was able to get away with things in the OHL due to his impressive physical tools but I really don't think Romanov has his level of offensive skills. I don't think if you put Romanov on TB he would put up 40 pts at 19 or even 32 at 20. Granted we'll never know but that is some very solid production for a young blueliner. Of course he benefited from being on such a skilled team and I'd take Romanov over Sergachev in the defensive end by a lot.