In ToT, most of the fights require so much spreading out, and so much sustained movement that Healing Rain loses a huge amount of effectiveness, even in 25 man raids. Chain Heal is too range limited for 10 mans, and too slow casting, expensive and inefficient even when it is used under ideal conditions (hits 4 targets with Riptide on the initial target). Glyphed Riptide spam is not really an acceptable option on most fights either. It doesn't provide competitive output and is too mana inefficient to be sustained.
I realize that it is early, but the initial results from ToT WoL back up the perception that Resto Shaman are way behind.
To put this in perspective, there is a larger gap between the 5th highest output healing spec and Resto Shaman than there is between the 1-5 healing specs. On top of that, Shaman are dead last on 11 of 12 fights. The only fight where they are not last (but are 5th of 6) is Jin'Rokh, which is the one fight where there is a significant amount of raid stacking, and where you would expect us to dominate (but we clearly aren't). The gap in 10 man is even more significant than the gap in 25 man.
I realize that Shaman have significant raid utility, and you probably will bring one in most any 25 man raid for Mana Tide/Purification/Stormlash from a healer. However, at these output levels, a Resto Shaman is essentially being brought as a token "buff bot", which isn't at all fun for the people playing that spec. On top of that, it isn't like specs with significantly stronger throughput don't bring significant utility too. You could argue that Holy Paladins bring as much or more utility.
I think that both Shaman output and mechanics need to be seriously looked at given the way they have decided to design the fight mechanics in T15. It's probably too late to make real mechanics changes (and the class design team dropped the ball big time by not doing this during PTR testing instead of rushing the patch out). However, an overall output bandaid increase is probably needed for the spec to remain viable this tier.