Is bubble spam feasible with the current regen numbers in-game? For example, can you actually cast PW:S 81 times on Maloriak in T11 gear?
Whether or not bubble spam is feasible depends on what is meant by "spam." In the post in question I divided the total healing that I did on an actual kill of various bosses by the approximate size of a Power Word: Shield from patch 4.0.6 to get the number of shields I would cast to contribute the same amount to the fight. This is a naive comparison to see whether shielding is an affordable way to heal, but is pretty far off the shield spam that discipline priests did in Wrath.
Maloriak has a six minute enrage times, and will almost always take pretty close to six minutes to win since for the foreseeable future we will need to wait until after the second green phase to put him under 25%. I'm almost in tier 11 now with an equipped item level of 356, so I'll just use my stats as a decent approximation. With all raid buffs up over six minutes I would have the following: 124k mana pool, 196k from in combat mana regen, 45k from replenishment, 11k from hymn of hope, 10k from a potion and 37k from shadowfiend. I'll talk about Rapture a little later. That's 423k mana. The cost of shield with Mental Agility is going to be 4626, so 81 shields will cost 375k. Clearly 81 shields in six minutes is doable with a lot of mana to spare.
Actually spamming shields, in contrast, is totally out of the question. Thanks to Borrowed Time I could in theory currently cast a shield every 1.17 seconds. That would be 307 shields in six minutes, costing 1.4M mana. That would probably be possible to 11 restoration shamans in my raid, but not under any plausible circumstances.
In reality using just shields to heal would almost certainly be wrong in a 10-player raid. Not casting Penance - especially now that we know it is also getting buffed - seems like a big mistake. It seems, however, that going from 4.0.3 to 4.0.6 a discipline priest will be able to match their old output using only shield and still have mana left over to do healing after that.
Does the inflated absorbency of PW:S mean this it will take a little better timing to get a rapture proc every 12s?
I do think that the larger size of the shield will make it harder to predict when your Rapture procs will come in some circumstances. Without predictability, you will have the choice of casting early and possibly "wasting" the chance at a proc or casting late and accepting that your procs per minute will be closer to 4 than to 5.
That being said, bosses hit pretty hard and a lot of the time their AoE damage will break shields. Halfus will do multiple Furious Roars followed by a Shadow Nova. Valiona does Blackout on the ground and Twilight Meteor in the air. On Maloriak breaking a Flash Freeze or taking a single hit of Scorching Blast will break shields. Magmaw does spew and spit at the same time. Shields won't be broken when a boss merely glances in your direction, but they will still be broken by randomly targeted attacks and most AoEs.
I think that actually getting a Rapture proc close to every 12 seconds is very hard. I used to get them close to every 14 seconds in Wrath, and these days I only cast shield every 15 seconds at most. But in 4.0.6 you will no longer need a Rapture proc to make shield worth casting. It's going to be one of your most mana efficient heals - and will totally blow competitors out of the water in terms of throughput - and discipline priests will go back to casting it a lot. It will be the heal of first resort instead of a weird mana return ability. Casting more shields should net more Rapture ticks, even if they take a little longer to break. I think we'll see Raptures between 13 and 14 seconds apart fairly consistently just because we'll actually be casting shield a lot. In the six minute fight above, that would be about 191k mana - over 500 mana per second and a great use of three talent points.
Do you think that Disc would fit in better with the current design philosophy if PW:S had a lower benefit from Mastery, but DA and maybe even PW:B got (better) use of the stat?
I honestly don't know what they could do to discipline mastery right now to fix it. I seems like they just need to come up with a completely new idea. It's too bad they already gave a healing-creates-shields mastery to paladins, since that would have made sense for discipline priests. Like other masteries, they are presumably want to keep it all down to a single number, so I doubt that affecting shields one way and aegis another way is in the cards, but it might be a good way to handle things. Unfortunately, the bonus to aegis would have to be over the moon to actually make the stat good for single target heals. If paladins get 1.25% of their heal to a shield every time, then it seems like discipline priests would need about an 11% boost to the effectiveness of DA per point of mastery since it is a percentage boost to 45% of the normal heal size less than 25% of the time.
I could brainstorm up some ideas for mastery for discipline priests, but it would just be a wishlist or trying to pretend I can comshow backseat develop the game I'm sure things will look different a year from now, but for 4.0.6 I think discipline priests should just enjoy being overpowered.