Currently willpower scales very poorly, which is the root cause of the problem. There's no real trade-off for stacking defenses, because willpower is such a bad stat it's barely worth considering.
As I noted in my earlier post, base healing should be reduced and willpower should be made to scale better.
The result will be less tanky healers, because like all classes they will have to invest in their primary stat. Additionally, by making willpower a stronger stat, there will be a reason for healers to stack more willpower at the cost of survivability, thus introducing risk-reward to the healer class.
So while this would be a nerf to healers, it will open up more build options for skilled healers who are prepared to forego high defenses and get greater healing in return.
By making healers less tanky overall, they will be more susceptible to harassment and focus-fire, and thus additional tactical options are opened up to break through the healer/tank safety net which in my opinion is the main benefit of this approach: more tactical options.
As to the exact numbers, I don't know. My suggestion would be to avoid rash changes.
