After the 5.1 patch, I spec'd to destruction with improved voidwalkers. I was amazed at the difference in void tanks! They now hold aggro! I ran stonecore (at level) up to the last boss but couldn't deal with all the adds. I cleared Vortex P (at level) with no problem at all. I fracking LOVE my warlocks now! They are what awesome sauce is made of.

Edit: The stonecore mobs are a bit (couldn't care less about your stupid voidwalkers) but targeting each one with a different void seemed to help.