He's great in the offensive zone, no doubt. He's got good hands, can pass well, and has a great shot.
He also has a lot to work on, IMO. The defensive game doesn't come natural to him; his defensive instincts/awareness need to be improved. You'd like him to be more physical, especially due to his size. He also turns the puck over more than you'd like, but that can be due to trying to create offence on a poor Soo team.
As for the WJC, it's still a long way away. There's up to three returnees on defence, and eight defencemen that got invited to the Dev. Camp, that could be back next year.
He's got a ton of potential, and is looking like a great pick.
I've seen great strides from this year in the aforementioned areas of concern you mentioned.
The Greyhounds have given up 45 more goals than they have scored. And this is reflected in the fact that like 85% of the team is in the negative for their plus/minus. His +14 shines even brighter, when you take that into consideration. When Sproul is on the ice, the Hounds are a +14 team, and when he's not, they're a -59 team.
His defensive awareness has improved considerably. He looks more comfortable in the defensive end that he did last year and he's really learning to use his long reach and physical gifts to play forwards off the puck on the rush. He's also increased his physicality, although he's still not a punisher. But he's definitely using his size more aggressively this year, especially in the corners to win loose puck battles.
The one thing you didn't mention is his skating and puck rushing ability and the improvement in his ability to create offense off the rush. Last year, this was a work in progress. The majority of his offense was created from manning the point (especially on the powerplay) last year. This year, he actually only has 20 of his 45 points, and only 8 of his 18 goals on the powerplay. He's much more aggressive in bringing the puck up ice, and he's following through on the rushes and creating offense from them. He's certainly not afraid to go deep into the offensive zone. But he's not lazy either and with his great skating ability, he rarely gets caught up ice.
Quite frankly, if this kid wasn't playing up in the Soo, and wasn't on a team fighting to stay in the playoffs, he'd be generating WAY more buzz. Put him on the blueline of the London Knights this year and it's a different story.
I'm a tremendously huge fan of this kid and love the progression he's shown this year.
