How are you computing your DPS "On the tanks", as opposed to overall?
"On the Tank" dps is not that relevant. It is overall dps for the encounter that matters. If you stick a Dirty Fighting or Lethality Gunslinger/Sniper to clear an add, most of his/her DoT damage is lost in the fight. Ideally you want the last add in the shield dead before the transition phase is over.
In order to beat the enrage timer, you will need to push the dps to extreme perfection in when and where to apply the damage. A good mixture of burst and DoT Class's are required. Burst classes can put out decent damage AND push the transitions phases themselves better. DoT class will be able to sustain a good amount of damage on the main boss. There are balances here.