Disc is extremely manaeffective now, I finished hc Shannox on maxmana again. Most normal modes I also exit at full bar. While topping the hps chart ofc
Keys to disc manaregen:
Int >= spirit from a regen standpoint. Yes.
3/3 mental agi + inner will (I only have 1/3 to have points for survival talents and still can manage)
Rapture proc as often as you can. This means you must break a shield every 12s or as close to this as you can (in practice, breaking a shield every ~17s on bossfights is good). Install a rapture ICD tracker addon if you have to and do not reshield the tank just before the icd is over.
Combine SFiend+Hymn of Hope if you can.
Spec train of tought 2/2 and macro Inner Focus to GH/PoH at least.
SoS 2/2 is not needed, you can remove 1 point from that pretty safely.
2pcT12 is good, yes.
Glyph of flash heal is poop, as is flash heal itself. Try not to use FH, it's only good if the tank is in imminent danger and penance is on cd. Use the PoH glyph instead.
Don't use renew. Only if you have to move and the target has weakened soul on him.
Don't pop PoM on cooldown unless most charges WILL be used. That's rare and mostly only happens during periodic aoe.
Power Infusion yourself when you know you'll need to spam poh/gh extensively in the next short timeframe.