In order to try something new, I went and did a raid as 53/0/18
Basic idea behind it, is that you spend about 20% of your raid time doing about 20% of the DPS a Mage would do, via SW: P and Devouring Plague. You lose some healing power, mostly through the lack of Crit and Crit Scaling, but you gain 3% Shadow hit and 10% Shadow Damage. You keep the very strong PW: Shield as Disc, Penance is still a life saver, and you got Flash Heal to help top off raid members. Keep ProM out there on every cooldown. Mind Flay is vastly superior as filler spell over Smite.
My experience so far, with 1 complete Naxx run and Malygos, is that this spec right now, has no obvious potential for DPS, you will be lower than Tanks. Healing wise, it both feels worse and shows worse than Disc/Holy. If Blizzard wishes a Disc/Shadow spec to be a serious contender, they have to fix some of the deep Disc talents to help damage output. I would never enter new content with this spec, but it is a good spec for farm content, and solo play.
Some things to note:
You need 14% hit with a Shadow Priest/Boomkin in raid.
Crit is mostly worthless with this spec. You want Spell Power and Haste.
I had 0 mana problems as DPS, had a bit worse Mana Regeneration as Healer w/same gear compared to Disc/Holy.
Mind Flay does not seem to eat Borrowed Time. Nor do DoTs. 25% haste on a 1.5s cast is 0.3s saved, 0.6 on MF, so PW: S -> SW: D -> DP -> SW: P -> MF actually takes 7.5s to finish. Without the PW: S it would still take 7.5s.
Mind Sear is pretty good damage in this Spec.