I am running holy/shadow dual speck. I maintain two gearsets of course. One is pure mana stacking for randoms, the other pure sp stacking for shadow pvp. As holy I am running just under 9k mana, which I assume is decent for this level (67). I have yet to see anyone with more mana than myself in a group. I have no issues will healing abilities, but I have been stacking straight mana to heal since 15.
My question is this, should I continue stacking mana till 80? If/when should I start to phase in some sp to balance the two, before 80, if at all? I have 340ish sp in holy set and 550ish in shadow set, but my shadow set has about 2k less mana.
I know this is a sort of fragmented post so forgive my noobness please
If you're healing groups fine then continue to do what you're doing honestly the game while levelling isn't expecting much of healers. I'd say take items with more intellect/spirit/mp5 simply because it means you can go longer without having to stop to drink, which ultimately completes the dungeon faster and levels you faster.
Don't turn down a piece that has a ton more spell power however, sometimes not having to heal as much because the heal is so big, is better than having more mana available. The best you can do is try and get a feel for it. Luckily enough once you're around level 60 and up, an upgrade is usually an upgrade in every stat and there is little decision to be made.
I'd also recommend disc for healing instances, shields are really great, you just have to know the instances a bit more to know when people are going to take damage. But it is definitely more mana efficient. And penance is great!