Unless you state it up front, you are in the wrong.
Maybe he was at 1600 DPS under normal circumstances. There are plenty of reasons for someone losing 300 DPS. Maybe there was a lot of melee and he spent time dodging spikes or maybe he was avoiding the clouds. Standing the cloud just to get a little more DPS isn't smart. If 1600 DPS is his max then even a little disruption will drop him a fair ways.
It isn't right to have limits imposed after the fact. Were the healers and tanks subjected to the same scrutiny?
Ideus, Blood Elf Death Knight
Officer, Tears of the Phoenix, Ysera US