Hello,
I switched from an affliction + 3/3 improved voidwalker, to a dark pact drain tanking build at lvl 42 (lvl 50 now). I realized my kill speed was kind of bad with the voidwalker, and it failed to keep threat from just regular DoTs and wanding. It worked very well though as long as the Gravestone Scepter from BFD was relevant, until early/mid 30’s.
Switching to draintanking, kill speed has probably doubled. Downtime has increased slightly and i usually kill mobs right when the initial corruption is finished.
However, I would say that survivability as with draintanking is a little bit lower, so you gotta pull out that VW for elites or other hard pulls.
In my opinion, Dark Pact is crucial for this build, although the skill itself is quite underwhelming as it only transfers 150 mana. You gotta get an easy key binding for it and start tapping whenever given the chance. At times, you need to turn of that auto-cast lash of pain as well. You can play around with having imp isntead of succubus, but i do believe that dark pact requires a succubus.
Also, spellpower is, if possible, ever more important with this build than with VW/wand build. As you’re dishing out even more spell damage, which really feeds into your mana efficiency.