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Since miracle specifically states its healing threshold, this might be a good case for an exception. Particularly since thats a pretty nasty abuse interaction.
Its really only an issue with the upped version anyway... otherwise you just end up with +1 healing
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I like the overhealing -> extra maximum health effect. That is actually more useful than the double healing in some ways. It makes heal decks a lot more resistant to OTK decks.
The cost is definitely quite high here. I think I might pay it for the upped version to get the bonus max HP, but for the unupped it would probably only be practical in a handful of cases.