This change was 100% required and anyone who didn't see this coming for the last few months is delusional. Disc is around 20% stronger than every other healer according to aggregates of raid parses and has more mana longevity than every other healer. These stats are with the vast majority of guilds still working on progression content; as people get into farm mode and more parses are coming from bosses that are on farm, Disc is going to become more and more ridiculously ahead as absorbs continue to scale with gear and effectively snipe more output from every other healer. Frankly, they should hotfix these changes in.
They just flat out dropped the ball and overbuffed Disc at the start of the expansion because of the amount of excessive whining from priests over how they performed at very low gear levels. The reality is, had they left it alone and people had been able to wait until they were at a reasonable basic gear level before crying bloody murder, Disc probably would have been been properly balanced.
The issue here is that they are nerfing our regen, they're nerfing spirit shell (which should net a ~15% healing reduction) and they're now hitting us with another nerf that will literally be in the neighborhood of 20-30% healing reduction. Now, by your own (compeltely accurate) numbers, this nerf is overkill, and that's all I and a few others here are saying. I think DA being brought back down to 30% instead of 50% would have been a fine change. Combined with SS no longer interacting with mastery, it would have netted us a nerf of around ~20%, exactly what you indicate we need.
Edit: to be clear though, I'm certain this is not going to stick. I think the PW:S/Rapture change is intended (and necessary, our regen was ridiculous,) but the PoH change is not.
No, 20% would not be enough. That would only be enough to bring you down to the level of the 2nd highest healer. The reality is, the nature of absorb heavy healing is to be properly balanced, Disc really needs to be at or near the bottom in terms of throughput. Preventing damage is far more valuable than healing damage up after it has taken place, so it still would not be reasonable for a Disc priest to do the same output as say a Holy Paladin.
On top of that, you have to consider that the nature of the Disc spec means that it scales better with gear than any other healer, so it needs to start the tier lower. Frankly, a 30-35% nerf over where Disc is on live is probably warranted.