Using enmity combo sucks but its not going to single handedly drop you from 4k dps to 2k. For enmity theres really 3 levels of enmity for raid bosses.

EZ mode. Nin+2tanks. Nin facilitates pull. 2nd tank starts circle shirk. GGEZ.
No Nin+2 tanks. Probably need to do suboptimal pull then circle shirk. EZ
Hard mode. No nin. OT is an idiot, DPS are good. Suboptimal pull. Scatter in enmity combos to stay afloat. Number depends on type of DPS (enmity shed abilities) and how good/geared they are.

Even with no help you shouldn't be doing half the DPS of your ideal situation. If youre capable of 4k then your tank is 340ish and pretty good. The only way to do 2k in that setup is spend the whole fight which is never necessary for enmity, only survival.

If your tank DPS cuts in half then that's his fault. Not the ninjas. In the early days of this tier I went from like 3600 to 3200 when I swapped from OT to MT and had no shirk and no ninja. Its really not THAT dramatic.