From looking at logs, it seems like the shoulders are theoretically nuts now on fights without constant damage auras. As I recently swapped from my DH (couldn't stand the 7.1.5 rotation), I have neck, shoulders, and belt.
Basically I'm thinking for raiding my setup will usually be neck/shoulders, whereas for high level M+, neck/belt. I did test out the belt in a raid last night, and you certainly can notice the mileage you get on your lifebloom (goes from ~5% total healing to ~15%), but I still think the shoulders were overall better, unless you really have the need for single target healing. Mostly I think the belt was just causing the HPal to overheal the tanks even though the numbers looked good. The shoulders also have a nice secondary effect of helping us conserve our mana, and making pre-hots much, much more efficient.
For M+ I think it's harder to tell. The lifebloom effect should really help keep the tanks up on high level M+, which is usually the hardest part. I don't see the shoulders being particularly effective, since mana constraints aren't really an issue, and it's typically pretty easy to keep double rejuvs on everyone.
Thoughts?