I don't think he is at their level, that is about as elite as it gets for a young defenseman, but definitely somewhere in between which is perfectly fine for a late 1st round pick.
I think we can easily succeed with Joker and Boqvist in the top 6 and load up and land a Mark Stone/Panarin in free agency.
My issue would be locking up another defensemen (Seabs, Keith, Murphy all signed) to a long term deal (no matter who it is, that is what needs to happen to get these guys..John Moore got 5 years IIRC) and blocking another spot for our young kids. I totally understand they all are not guarantees to make it, but you have to keep room open if you feel they will be impact players which Management seems to feel at least Boqvist and Mitchell will be...
Beaudin, Krys, Hillman, etc are all solid wild card guys who can potentially be top 4 dmen, but I am not holding spots for any of them...but Joker, Boqvist and Mitchell...yeah, I would make sure there is room here for them to make this team and show what they can do.