The point, though, is that 900 damage mitigated to one person and 2400 damage mitigated over 8 people are not really comparable because of AOE healing.

Far more apt is comparing that 900 damage vs. the 300 damage. Adding 300 more damage to a 2700 hit is way less threatening than 900 damage added to a 8100 damage hit. That 300 damage probably doesn't expend an additional GCD and 3k isn't all that life-threatening. A 9k hit represents a much more serious threat of dieing to a tank.