But it's not mitigation that's holding WHM back.
For as long as 2 of the 3 healers can increase raid DPS... people are always going to want to use those 2 as soon as they (comfortably) can.
Give WHM more mitigation... you make the job a little stronger in progression content. Perhaps guaranteeing a slot on next tier's progression wave (is that the goal?). But, the above will always be lingering over the job...