Has anyone even tried this…?
It makes a lot of sense to me for pvp anyway…
I haven’t played my priests enough in 8.3 to have multiple gear sets to test it out, but I’ve wondered this same thing. Defensive penance and shadow mend would gain a nice boost, and rapture shields could be noticeably stronger.
Only thing with mastery though is it contributes nothing to dmg. With that missing healing from dmg boosting stats, it’s hard to say what the difference in healing output would be. Interesting idea for sure and one that I’ve often thought about!
Idk how mastery rating translates to % of mastery you gain but if it’s anything like mine and you have 130% base mastery… your shields are 130% stronger, atonement is 130% stronger so you’re healing a lot more while in CC WITH atonement+shadow word pain on stuff.
You’d be forced to hard cast far less often and far more mana efficient…
I think this person’s thread in the BG forum was focused on absorbs in PvP Disc Priest Healing/Absorb burst in BGs
Mastery rating is different for every spec. https://wow.gamepedia.com/Mastery
Disc priests get 1% for every 53.3 rating assuming those ratings are up to date.
that would be me 
Im at 61% Mastery at present, which makes for very strong Absorbs and direct heals on anyone with Atonement. With a few Mastery procs, Ive had it bump up to over 130%, and then I see the likes of a standard PW: Shield for 70k.
Versatility scales a lot less than Mastery for Disc, so the damage-to-heal ratio would likewise be a lot lower. That said, stacking Vers is probably better for Arena where you can add more damage pressure with teammates.
Whereas in a BG where there’s more healers about, Disc’s extra damage doesn’t really make as much of a difference, yet stronger direct healing and absorbs is required due to more allies taking more damage.
Mastery stacking balanced with a decent amount of haste, crit, and going for all DPS talents makes for a laughably fun time in BGs. I’ve had up to three DPS beating their heads against a wall due to spirit of preservation minor, 3x depths, and damage healing from atonement. I whittle people down with healing from damage heals, if they block holy then my shadowmend does back to back major heals, and worst case I use standstill to buy me time. Makes you very tanky, and pisses people off in BGs lol
its prob broken. Id go for it.