Vol'kaal is going to suck on M+. No two ways about it. That dungeon and the Underrot are going to wreck keys as you've stated. The hunter trap I'd be careful of falling into is that on raid-wide damage they're inferior to warlocks for simply two reasons. 1) Healthstones and 2) aspect of the turtle is nowhere near as good as Unending Resolve. If you get pressed hard as BM it won't matter what you do.

Also CC options are better on warlocks. Keeping some of those nasty mobs permanently feared and stunned is ideal. I'm still working out how to do a lot of things sans my naga. I really miss it. But it'll get there and one of those things is how to handle a stun rotation with shadowfury and cap totem.

All in all still happy I went with warlocks. Though we'll see how that goes.