The results are in. We've killed Argoloth so I have some preliminary examples of real raid healing. Did I meet the developer expectation of 50% healing from shields?
The right answer is not even close. World of Logs shows me getting 10% of my healing from PW:S and 25% of it from Divine Aegis. Their Divine Aegis numbers seems a little sketchy, however, since if you add up all the critical heals and Prayer of Healing heals I did in the entire fight and take 37.5% of that - my current return on Aegis thanks to mastery - it doesn't reach the amount of aegis absorbs that World of Logs calculated. In fact, I applied only about 80% of the shields that they say I used, and I know I didn't use them all. It wasn't until halfway through the fight that I clued in to the correct healing strategy to avoid wasting Aegis.1
So my real healing from shields was around 31%. On that fight I can get 100% shield usage if I play correctly, so we can use that number as an accurate representation of the amount of shields I would put out on a fight.
The reason that this matters, as I've noted before, is that mastery rating is tuned according to the amount of shields that discipline priests do. Mastery rating increases shields, so how powerful it is is dependent on what fraction of your total healing comes from shields. However, another change that has been made since I last did analysis on this is that Prayer of Healing always triggers Divine Aegis. That means that mastery is significantly more powerful than it would have been while critical rating is significantly weaker.
If I had 179.28 more mastery rating, I would have increased by mastery bonus from 25.12% to 27.62%, thus increasing my shields by just about 2%. That would increase my total healing and shielding by about 0.6%.
179.28 crit rating would have increased by critical strike chance from 19.06% to 20.06%. That means that 1.24% of my non-critical spells would instead have been criticals. Since my non-Prayer of Healing heals totaled around 166.6k, that would mean critting with an additional 1399.5 worth of heals. My effective critical multiplier is 1.5 plus divine aegis. Aegis is 30% times plus the mastery bonus for 37.5%. This gives a total critical multiplier of 1.5 * 1.375 = 206.25%. That means I would get an additional 1399.5 * 106.25% healing by critting with those heals for 2186.
Now for my Prayers of Healing, I get the Divine Aegis on the base even if I don't crit. That means that I only rack up 50% * 1.375 extra healing on a crit, or 68.75% of the base spell. While I did overheal with Prayer of Healing, this is largely due to the fact that the rogue was standing behind rather than in front of the boss, and so prayer of healing hits on him were largely totally overheal. I did 546940 healing with non-critical Prayers of Healing. If 1.24% of those were crits and those crits added 68.75% more to the total healing done then that would contribute 4644 extra healing.
The total extra healing from a 1% gain in critical strike chance, then, would be 6830. With just over 2M healing for the total fight, this is an increase of 0.33%.
These calculations show Mastery as a far better stat than critical strike rating. Of course this is contingent upon casting a lot of Prayer of Healing. What would these numbers look like if instead of making Prayer of Healing always trigger Aegis, they had simply decided to give Discipline Priests a talent to increase its effect of Prayer of Healing?
The buff for Prayer of Healing is contingent upon not getting criticals. If you get a critical heal then the buff does nothing. Let's say I averaged my critical strike and mastery ratings to eliminate my current reforging bias. Then I would be critting 17% of the time and my mastery would be 12, giving me a 30% increase to shields. At that point, the buff to Aegis to always apply to prayer of healing would be boosting the effect of 83% of my Prayers of Healing by 39%. The overall buff to Prayer of Healing, then, would be about 32%. What if they had just put in a talent that increases the effect of Prayer of Healing by 32%?
Then my shields would be only 19% of my heals, so 1 Mastery would be a 0.36% increase in healing. 1% crit, in this case, would be a 0.56% increase in healing, nearly three times as much as Mastery.
It would seem that Discipline priests require two different gear sets. One that is geared towards critical strike rating for single target healing and one that is focused on mastery for AoE healing. For now I might try to even out the two stats since I don't know what raids will have in store for me yet. Ultimately, these quibbles probably make very little different since both critical strike rating and mastery are fairly weak. In the best case scenario, if all my new secondary stat points went to mastery, an entire tier of new gear would provide less than a 2% healing increase from these ratings.
1. Meteor slash happens every 17 seconds, and Divine Aegis has a 12 second duration. Since meteor slashes alternate between two groups, a group will take one only every 34 seconds. Thus, if you spam three or four Prayers of Healing on the group when the slash hits, the Aegis will wear off before the next slash. If you space your Prayers of Healing out over the 34 seconds then you have the full Aegis from all of the prayers roll through to the next slash. You just have to battle against those Wrath-tuned instincts that tell you that if anyone is below max health you need to spam until they aren't. This should mean that a discipline priest can take care of meteor slash damage by themselves.