Let us take the current state. You have 5 healers and one mixed class and statistics for 7 different bosses. The healer does heal and the DMG output isn't there. It is easy to compare these 5 healers over 7 different encounters and start tweaking around to lift some up, that are behind and tune manacost here and there. Add some utility, to make the difference be resonable and you have done your tuning.
Then you have your one mixed class, one with an very high skill cap and therefore with a huge range of performance. The difference between good, medicore and bad on this spec, has a bigger impact compared to other classes. A mixed class you never had in over 11 years of existing of your mmo. The important part is: There are several things that are hard to "put a number on". The scaling with raid size and possible attonment targets are one thing (10-13 Man raids are very common). The other thing is, how much dmg equals how much heal. But it is hard, I cant really compare apples with melons.
The question is: What is a disc, is he a DD that helps out with healing in certain situation or is it a healer, who adds some DMG. (The problem here is: In the current state, people want to compare the numbers with numbers from different specs and classes).
If you ask me, I would change the Attonment mechanic. The current state is: Here you have attonment for 15 second, you have a strangely powerfull hot for the next 15 seconds (If your disc does damage). If your Dmg input exceeds the hot, you are going to die, if you are full, yeah just a bit overheal for you to feal safe.
What could be done to fix it?
First: Attonment can stack, with the stack reduce after 15 seconds. So you can give someone a more powerfull hot. (add a limit to it, maybe 3). This will also helps out in dungeons with haste/mastery gear and wasting attonments will be reduced a lot. (Reduces the skill cap a bit)
Second: Your Damage and Heal/shields (not attoment heal), will also apply to your attonment targets (maybe tune the current %). This gives you the opportunity to go full force on healing and just use penance and dot dmg to heal, or go without Dmg at all.
Third: Disperse/Collect attonment. Should be off GCD, very small mana cost and disperse/collect all attonment stacks to random (low health) targets or 3 to your target. This will help you with more reactive healing (disperse/collect into healing/DMG) and reduce the ridiculous overheal, when a lot of your targets get sniped.
Four: Cap the manacost on plea (6-7 stacks)
This way the introduction to disc isn't hard to learn and hard to master, instead it is medicore to learn and hard to master. Where the easy part is, where you just use your heals and handling your attonments and you are fine, mastering is the part where you add the dmg part to it. So you play with 4-5 buttons at the start just for basic healing and add 2-3 damage spells when you are ok with the first part.
Sorry for going OffTopic and my bad langugae, not a native english speaker.