Blizzard has continued to rework Serendipity for Holy Priests. It has reduced the mana reduction portion by 20% instead of just 10%. The new tooltip for Serendipity:
When you heal with Binding Heal or Flash Heal, the cast time of your next Greater Heal or Prayer of Healing spell is reduced by 20% and mana cost reduced by 20%. Stacks up to 2 times. Lasts 20 sec.