darkness is a very good talent toi have !!!
Go for 3 points, reforge some haste into mastery, voila. And get 346 pants
That looks quite normal for 10 man, assuming healers of approximately equal skill and especially considering you're tank healing in phase 1 and I would imagine to some extent in phase 3. Our logs look very similar, with me primarily healing the Nefarian tank (as Discipline) throughout the fight with some raid assistance after Crackles, I'm last on overall healing done, but like you I have significantly less over-healing than our Paladin or Druid.
At the end of the day, if the boss dies it doesn't really matter. It can be disconcerting constantly doing less healing and feeling that you're perhaps not pulling your weight, but part of it is class balance; something that's being addressed in the next patch. I can't see anything obviously wrong with your gear or spec, certainly nothing that would suddenly make you do 20% more healing.
it does ? name some