Your initial math did not count the dps time taken away having to deal with adds. I'm not saying its impossible (I'm quite certain it is) Its just not realistic for players of average to above average skill level. My main point is, if their goal was to focus on mechanics, then enrage timers should not be the primary "fail" condition of the encounter....not executing on said mechanics should be (I.E. Incenerates, adds, lightning, fire grates)
Edit: Enrage timers should only exist for 2 reasons:
1) To prevent stacking of healers (something that is not an issue in a 4 man dungeon)
2) To provide a raw gear check in a tank and spank type fight
Any other reason is just lazy design, imo.