We're at a point now whereby a decently geared Disc Priest with adequate Rapture management does not go OOM. It's simply too strong and it is going to scale far too well.
Compounding this is the buff to DA - and it also poses another issue: we're now able to spam PoH with little regard to its initial healing. There is no balance when the DA from PoH is able to shield a group for at least 125k, even if the initial heal does 100% overhealing.
Lastly, I truly believe that SS needs to be toned down slightly. I can appreciate the high skill cap involved with it, but being able to shield the raid for up to 2.5mil every min is really powerful. There's a reason why we're starting to see a trend of top guilds stacking Disc Priests. A reduction to the SS buff on the Priest to 12s whilst increasing the uptime of the secondary SS buff to 18s would nerf it slightly, whilst still reward Disc Priests for timing it properly.
Not everyone has the luxury of raiding HMs and for those that don't, I simply don't think it's fair or fun for the other healers to be twiddling their thumbs whilst Disc just snipes all the heals. Certainly, Disc is a class based on absorbs and it's going to happen to some degree, but Blizz has honestly overtuned us. I'd rather they address it now in a reasonable manner, rather than having Disc end up as Monk 2.0.