I just boosted one of my DKs to 100 for my DK, disc priest and 3 mage team. In his il640 boosted gear i was doing 33k on Kaz with a frost dk gnome sequencer macro ( just used GS to get him going). I'm sure if i spend time using ISboxer step+priority I could get a lot more dps out of him. My main team of paly and ele shams are in il700ish gear and do around 30-35k on kaz.

On another note my 5 boomkins do less than 10k dps in il640 gear lol! Anyway, Dks seem to be easy to get good dps out of while multiboxing.