A new change has hit the PTR for Disc Priests, although how it will really affect Disc Priest’s mana remains to be seen – people are conflicted about whether it is a gain or a loss at this time.
The change: Rapture now energizes the Priest for 100% of the mana cost of Power
Word: Shield When the shield is fully consumed or is dispelled (used to
energize the Priest with mana equal to 150% of their spirit). This is
the modified mana cost for Power Word: Shield, after being reduced by
Rapture’s passive effect.
There is currently discussion as to how this will really affect mana for Disc Priests at HowtoPriest