Stars went from 2.5/2.54 xGF/xGA (5v5 per60) to 2.71/2.65 from Bowness -> DeBoer. I sense the Marchment signing plays a role here, as he plays a high event offensive style.
WPG went from 2.57/2.76 xGF/xGA from Maurice to 2.68/2.69 under Bowness
Bowness has always seemed to me like a drills and motivation sort of coach, and he accomplished in WPG the same thing he accomplished in DAL, get the players to buy in to a layered defensive system. Hellebuyck clearly benefits from his teammates playing more predictably in front of him. But despite WPG's record this year, the underlying metrics show they are still fundamentally a Bowness team - low event and treads water at even strength - but I think the entire idea of his system is that sure you give up the same chance volume but you're giving your players better resources for defending those chances, while the offense infamously will "take care of itself" and as long as it does, the system works quite well.
I'm having trouble getting a grasp on DeBoer's coaching style, though. Defense has been the elephant in the room all year, and so far any attempt to adjust has caused the offense to dry up. Fall behind and let loose has been the recipe for offense so far. I doubt the players are executing the system to perfection yet, and that's probably why we're seeing that the players who are grasping the positioning requirements quickly are getting disproportionately high ice time (Suter).