Standard canon according to the priest forums and EJ during tier 11 was haste (12.5%) > mastery, then after the soft cap haste = mastery. Once holy priests started doing heroic tier 11, they veered more toward haste as they found it more effective. Stacking one or the other has so far depended on personal preferences and raid comp. For example, I always ran with a resto druid on 10 man and I found that most of my mastery hot was overhealing, making mastery far less effective for me than haste.

While I love Mr. Robot, it's best to take it with a grain of salt in regards to healing, especially when stat weights depend on outside circumstances such as raid comp and role (disc tank, disc raid, holy tank, holy raid, disc attonement, disc shield spam, etc).

@Revnah
Check out overhealing as well as your regular healing for Echo of Light. Mine had quite a bit of overhealing when I ran with resto druids, but it also wasn't as high on regular healing as yours. If it's kind of in the middle, you might find that keeping haste and mastery balanced would work well for you. I reforged all my extra mastery to haste since it wasn't doing me any good on the overhealing charts :(

@rebecca
The haste cap is only a soft cap, so it's still beneficial after 12.5% and far from "useless".