If a smite / HF crits, its associated atonement counts as a crit. An atonement crit gets a DA, just the same as a Heal or Gheal crit. Sure, mastery affects single target heals less than it does PoH.
The fact that you're taking and using AA/A shouldn't dictate that you're using single target heals more often than any other spec. It just changes your fill pattern. If you're smiting so much that mastery won't benefit you much, I don't agree with your healing strategy ><
Same answer as for everyone else to ask this question of disc. It depends on your healing breakdown. Top heals should be PoH/Aegis/Penance/Atonement, in some order similar to that with atonement less than 8% of your total healing. Mastery can still hold some value. Being AA/A grants you more mana for the fight, so haste is maybe the only one that's obviously easier to make use of.
Depends what sort of disc healer you are. Are you a main tank healer, or a raid healer? If you're tank healer, then best bet would be to get as much haste as you can as well as applying some mastery along the way. If you're a raid healer, balancing haste and mastery has proven to be extremely effective for me as I have reforged a lot of my spirit into mastery so that I can balance a high haste/mastery setup. If you ask me, crit is a worthless stat which should be avoided to reforge into. But that comes down to how you like healing in my opinion. I'm currently rocking it fine in HC Firelands atm with my setup if you wish to see: (search for Soshii - Laughing Skull - Horde) Sorry cant link due to new account :<
Yea, currently I am 2129 haste, 802 crit, and 634 mastery. I prefer to tank heal, and I hate for my spells to feel sluggish hence why I stack haste. I keep mastery the lowest because I only use bubble for rapture / oh shit moments or mechanics like rageface. So I feel I get more use out of crit.