Increases the HPM of any periodic effects like always. The only really relevant one is renew, of which the HoT portion can make up 20-30% of your total healing done in a typical fight. Lightwell and Sanctuary scale, but they're usually less than 10% combined.
The reason why Holy can get away with stacking haste is because it is incredibly mana efficient. I've finished fights at 50% mana when my cohealers were OOM. If you run into mana issues, naturally haste becomes much worse since it just kind of compresses the time it takes to do the healing, instead of actually increasing the total healing done.
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That's an interesting thesis.
Here are some calculated stat weights:
100 mastery is 1.18% healing
100 multistrike is 1.145% healing
100 haste is 1% healing
100 crit is 0.9% healing
100 versatility is 0.769% healing
If mastery does not benefit x% of your healing, then you need to multiply the weight of mastery by 1-x%.
100*0.01818*(1-NMP)
NMP = Non-mastery percentage. This is the sum of the hot portion of renew, sanctuary, lightwell (testing), and one other spell I'm not inclined to remember.
And this will give you your own value for mastery.