literally you say that even for a pure 40th lvl druid, the magnitude of his heal will be cut in half, and not only that, but it will take 10 rounds to reach that magnitude. I'm assuming the regen gained from these heals will not stack? I'm also assuming that even if you use a scroll or item to cast heal it still operates in this way, even if you only have one level as a druid (in the same way people would always take 1 level of healing domain cleric to get the 50% bonus on Moth's). Seems like a pretty raw deal to me. It would also completely discourage multiclassing in a couple of druid levels.
I do generally like the idea of there being two kinds of healers. One kind that can give you quick bursts of high magnitude healing when you're almost dead, and another kind which is targeted more towards keeping you healthy to begin with. Really, all I'm saying is that the numbers seem a bit low for that second kind to be effective.