I use castsequence and it's fine, not the best, but it works. All my DKs are blood and that is for the stacking blood aura and worms. They dont require much healing with all that and rune tap. Your dps sounds about right, honestly with all the positioning and running around mine rarely break 1500. It's sad that you cant see the massive dps of an all caster team, but we make melee teams for the survivability and fun factor, eh?

edit: Also another thing I found helps A LOT is breaking up your dps keys to 2 types: Rune generating and Rune consuming. The problem with castsequence is if you hit Death Coil and you dont have enough Rune then it will never fire off and your guy wont be doing anything but white hits. Im not at home, but my sequence was something like Icy Touch, Plague Strike, Blood Strike, Blood Strike and the another key for Death Coils and Oblits until you are out of rune/diseases. Just alternate those two keys.