When the round of 25M buffs came out a few months ago, resto shamans were left out. And healing rain has performed a bit poorly for resto shamans this tier because so many of the sites had a lot of movement and a lot of mechanics it required the raid to spread. Fortunately, Healing Wain will get a buff in 5.4 for shamans.
For 25-player instances, Healing Rain will heal for a reduced amount when healing more than 14 raid members. It remains the same in 10-player instances, healing for a reduced amount when healing more than 6 raid members.
This is in addition to to the previously announced buff where the radius of healing rain was increased from 10 yards to 12 yards.