OK so here are some numbers. My disc is only 819 ilvl and has all of the DPS artifact traits including Sins and Dark Side.

Duration: 2m
Target: Raiders Training Dummy in Orgrimmar.

1. Straight up DPS, no healing, no atonements at all.
Talents: Schism, Power Word: Solace, Power Infusion, Divine Star, Purge the Wicked.
DPS: 116k
Healing Done: N/A

2. Keeping up atonement on self only via PW:S.
Talents: Schism, Power Word: Solace, Contrition, Divine Star, Purge the Wicked.
DPS: 105k
Healing Done: 8.3M

3. Keeping up atonement on party of 5 via PW:S & plea.
Talents: Schism, Power Word: Solace, Contrition, Divine Star, Purge the Wicked.
DPS: 98k
Healing Done: 33M


What's interesting is that for #3 Light's Wrath did more damage than Penance with 22% of DPS due to the 5 atonements. In #2 Light's wrath was only 5% of DPS and Penance was 25%. The 50% buff to Light's Wrath plus the 5% buff to all damage due to the Sins artifact trait matters a lot in #3. Without those boost I'd have lost another 10k or so DPS.

I think in 880 gear these numbers would at least double. The healing output would be pretty insane once you multiply by 5. Maybe enough to justify have one or two of the priests go shadow.

One big flaw though I'm not taking into account with these numbers is spell pushback when there's a melee mob on you. Maybe a shining force rotation could minimize that, and the damage being taken thereby also reducing the need for blanket atonements (only using plea on priests being damaged, but that gets hard to micro-manage). If one priest was shadow you would pretty much guarantee they had aggro though by using fade on the discs and due to the pure DPS output of shadow (Single target and AoE), so putting up 5 shields on him would minimize pushback across the entire party. Now I'm getting super theoretical though and in practice none of this may work!