First: sadly, too many healers will actually let the tank go down to 20% in PUG raids. Statics are different, since such healers will not be accepted to be part of the group.

My point about Upheaval is not to try to get 910 often, but should the HP be too low, upheaval suddenly goes past decent and all the way down to a waste of BG. The raid wont fail because the war or pld cant do their HP% based attacks at full potency, but nor will it fail because the healer makes sure the tank is always over 75% HP. Healer aoe damage is ridiculously good, but ST damage isn't.