Personally, I'd prefer the Imp DS buff at least 1 tier earlier in the Disc tree.... I somehow doubt that Disc will ever be a truely 'great healer' spec... and even in PVP, they are fairly weak healing (but dang near impossible to kill..... /grin). The mana return from the healing, will 99.9% chance, be from effective healing only.... which will make it a lot less than most think it will actually regen mana....
But with the new downranking nerf, I think the majority of the healers in T7/8 content, will be priests + druids, with 1/1 or 1/2 shaman/pally (or other way around). For the most part, priests really have never had to do very much downranking, except maybe for tank-insta-gib spam fights (brutallus...... enough said
). Only time I've ever needed to downrank a spell, was for tank healing.... and with the new Priest 80 spell (was hard to refind, as the level cap hasn't allowed access yet), which grants the target 30% of all healing the priest does for 30 min, and with the new mana refund from overhealing, will be quite the compliment for CoH Tank/Raid healing