Here's my current Dark/Dark build. This was a recent project that was part of a SG leveling team. I haven't slotted any purples or winters yet, but it's stupid tough as is. I don't have capped defense to anything, but really haven't missed it. Strongly recommend you take a look at Touch of Fear, which is now an AoE attack (the fear and tohit debuff are still single target) on an 8 second recharge.
I tried this and really didn't feel like it was offsetting the end cost. I moved the Theft of Essence proc to Siphon Life, which I use as part of my attack chain, and hit Dark Consumption after using Dark Regeneration if needed.