He may be 6'1" 200lbs but he's still 18yo. playing in a league of mostly 17-20yo is fine I'd be worried if he couldn't move on to pro after his D+2
As far as situation comparables go I think Bo Horvat is a good one. Both were NHL sized in their draft years (Bo listed around 6'0"+ 200+ lbs in old draft combine articles), both big Cs known for their goalscoring, and both playing OHL in their D+1 season. Nothing wrong with it.
The problem with this comparison is that Dvorsky is nothing like Horvat. He's almost the exact opposite.
Horvat skates very well for his size. Took extra skating lessons very early on as a kid already with a former figure skater I believe. Dvorsky on the other side still can't skate at all. At least for a top10 pick his skating is very underwhelming.
Unlike Dvorsky, Horvat was never depending on his frame. I'd go as far as to say he should in fact make way more use of it.
Horvat is a highly respected two way Center who has always been excellent against the puck. Dvorsky is still rather lost against the puck. His positioning is in need of a lot of work. Kids are unable to expose him but against men he was mostly useless at best when his team didn't have the puck. Which was part of the problem because that's what he was facing. The teams he was on weren't exactly top teams and he wasn't playing with their top guys either. Of course the puck was mostly in possession of the opposition.
The problem is that Dvorsky can dominate the OHL by doing the exact same thing he did the last few years. He won't see the need to change anything and work on his weaknesses, change his game while completely dominating. The problem is that he has to do exactly that. Right now. Because he can't play juniors forever. As you say, he should be a difference maker in pro hockey next season. He's a top10 pick after all. The foundation for that has to be laid this season. While I respect the CHL for what it is and would agree that their leagues are the top junior leagues and the place to go for most kids including top prospects. Having said that, every player's situation is unique and I still don't think that for Dvorsky it was the right call to go there. If he wanted to do that, he should have went there earlier.