I'm currently doing old raids with my guild and eventual pugs in ToT normal, but what I've noticed comparing my heals with people with similar gear is that Atonement is very very strong and the right call in more than 70% of the time I'm casting.
I'm used to be top heals on most fights, and I don't even press Archangel or Spirit Shell anymore unless I know the other healers won't be able to heal people up, just because I'm too tied to the DPS race.
I like it too, I love to be the top heals doing 40k+ DPS, but imagine how strong this is in 10m. It's a press 3 buttons to be more useful than anyone else.
Compared with a Fistweaving Monk with similar gear I realized he was doing more damage than me, but still doing less healing. I was going around 40k dps and he was doing 45k, but my heals while DPSing were far better and he said everytime people started taking damage he was changing his playstyle. The more "changing" in playstile I was doing was to pop some PS:Shields and PoM after big damage and then using Divine Star to help him and the Shaman bring people back up.
I think pressing Archangel and doing some PoH should be more effective than continuing to Penance the boss on those particular times. If we're able to do more damage but our healing while DPSing are not enough to bring people up (but still effective in maintaining them for a while), we would pop Archangel more and do healing stuff a little more. But I don't believe a 20% nerf would acomplish that.
Instead of a 20% nerf, I would bring Atonement down to 50% healing and increase critical chance by a significant amount (15% - 25%). This way it would become our "auto Divine Aegis generator" that we lost when PoH got nerfed.
We would do a lot more damage, but that would be around the same since we are changing stances a lot more now. If the Priest is designated to press DPS, he would work as an off-healer, doing around 70~80% of a DPS damage, but healing for 20~30% of his potential. That's still good imo.
But it's only going to work right if they change Atonement "smart" healing to prefer players over pets. I would say it should prefer people with the most max HP, so we generate more Divine Aegis on tanks. This change alone would already be a big buff to Atonement, even with a 20% nerf, if it stops healing (and creating shields) on pets and start healing the tanks more it would be an overall buff.
The change in gameplay is that now Atonement is very strong in maintaining people up when there's no big raid damage, but once the raid starts taking damage and our DA is consumed we have to pop Archangel and heal people up with actual heals.
Our "actual" heals now suck, especially Heal and Greater Heal, I don't mean to use them either, very slow cast times and not "emergency enough", so I think Archangel should buff our other heals more, maybe reducing Flash Heal mana cost or PoH cast time.
This way we would cast Atonement spells for a while, with some PoM and PW:Shields to help keep people up for longer, and then pop Archangel, Spirit Shell and PoH/FH when big raid damage comes, or Archangel and PoH/FH to bring people back up.