I don't know about you, but I am in a 16/16H guild and we got our Sha kill January 15th. I don't know what "HM Guilds" you have talked to but from my experience with "HM Guilds" they do not FORCE people into playing a OP spec. They expect that person to play their class to the absolute highest potential. And Discipline just happened to be that spec this tier.
There are fights in 5.2 where disc will be better and there are fights in 5.2 where holy will be better. That is an extremely obvious assumption.
I can say that disc was not nerfed. Just because they removed DA from PoH and made SS less op does not, at least in my eyes, constitute as a nerf. If you understand the changes and are prepared for 5.2 you will be able to do the same amount of healing as you do now. I know this from PTR testing. I am not just pulling this information out of my ass, I have experienced it and I know what the disc changes have done for overall disc effective healing.