Healing and shielding aren't the same, though. I don't think there's an easy answer to this, but I'm wondering if you guys aren't making a fundamental mistake by lumping in those two mechanics together, even though at a glace, the added effective HP added to a champion doesn't really care if it's permanent HP or not, that's already accounted for by the abilities themselves.
For instance, if you're nerfing healing hard on the basis of healing reduction being prevalent, doesn't that make the shield nerfs harsher in comparison, considering there isn't grievous wounds to speak of (disregarding the atrocity that is Serpent's Fang)? Just food for thought, I'm very unsure what the result of this is going to be.
If it ends up being necessary, are you guys going to return power to the affected abilities or look elsewhere? Because that's another concern of mine.