Mastery hasn’t been a very strong stat for holy priests, particularly when there is a lot of overhealing going on (which there usually is, unless a guild is deliberately underhealing a boss). Blizzard answered some questions about the role of mastery for priests and if it is being reworked for WoD.
Any chance on HPriest mastery being relooked at for WoD? Maybe turn all the overheals from it into something?
Echo of Light us something we balance assuming it’s going to be more overheal than average. More of regen than healing in design. (Source)
It’s not really a choice when it maths out to be 25% or less of the value of other secondaries in typical 25H.
That only sounds accurate when significantly overhealing, and only if your goal is topping the healing meters. (Source)
If you’re not overhealing, and your goal is minimizing deaths, mastery can work well. (Source)
And when in the last 3 expacs has there not been significant overhealing most of the time even progression?
I’m referring to using more healers than you need. Doing so results in RShams looking weak, and Discs strong. (Source)
Sounds as though it will be another one of those things that will work well earlier in the upcoming expansion, similar to how priests were stronger at the start of this expansion too.