Comparison... Disc 5.1 AoE heals versus 5.2 AoE. This is based on one AoE damage phase, Imperial Visier Zor'lof in HoF, Force and Verve which (assuming all are underneath shields) is 60k damage per second per player for 10 seconds... 600k Total. Enough to kill eveyone, clothies 1.5 times over.
5.1.... 4 x PoH = 54k mana, 1 million heals. Sounds a lot, but that's only 100k (inc DA) on each player over the period as compared to Holy PoH which gives 75k. Plus they have spam renew 50k, CoH and Holy Word: Sanctuary. Won't worry about Lightwell (even though it will last through both phases) or DH because of their CD's.
You can pop SS for the first Force and Verve so ramp up 150k+ shields for this and heal through to the tune of another 50k per cast (inc DA) for 100k per person across the raid. That's healing just under half the incoming damage, the other healer has done less numbers but similar and because we were at full health when we started, we're looking great. No drama with the first one... easy.
SS is on CD from the first F and V, PoH is going to heal for 30k per person per cast. This amounts to 60k heals to each person over the 10 second period. Thats 10% of the incoming damage... Awesome. Clearly this is not feasible. So we pop AA and increase the healing to 75k per person. Nowhere near enough, so we pop PI and get another PoH in taking it to 112.5k heals on 5 members, the others with only 75k. We're asking a lot of the other healer here.
What can I do to try and get myself to say... 100k heals on everyone over 10 seconds? That's around 16% of the incoming damage.
1/ 10 x PW:S? Over 137k mana, 86k heals on everyone, takes the full 10 seconds to cast. Nope.
2/ 1 x Penance and Smiting away (say 6 of them). 120k plus 180k. 300k, average 30k per person. Nope.
3/ Binding Heal x 7. 840k heals, 84k each. Not enough and oops.. 153k mana. Nope.
Iv'e tried mulltiple combinations of all these taking into account the HPS, Mana cost and cast time. Not a one is viable and/or amounts to 100k on 10 people over 10 seconds. Sure, I didn't include PoM, it's a one off during it, or two if you cast it beforehand, so add 20k healing to everyone assuming best case scenario.
In the case of Force and Verve, you can get through if the other healer pops a CD on round two, but this is just an example of how weak Disc Priest AoE heals will be after 5.2. What about phases such as the final Imperial Visier Zor'lof phase, Earthquake, Draw Flame, Titan Gas etc?
How scary is this? Or are my maths blatantly out?