I think you need around 21k party DPS to clear OS2 (at least our groups had a bit more than that on our first kills). Assuming your tanks do a total of 3500 (and why would they do more, it's not their job to DPS after all, right?), that leaves 4375 DPS for each DD assuming the healers do nothing. Can you honestly say it's fair to ask each of your DDs to push that much DPS while the healers do nothing but heal, and that if the group would meet enrage in that situation it would be the DDs' fault?