DH scales with haste, easy to spot and to test with indeed Altairus and Cookie in DM's.
Same for hymm of hope btw.
Also about the tank spec, the Atonement spec u have as raid spec. IS also good for tank healing, i use that (2 points different tho) spec as tank healing and my smites hits for more than heal can, its more effective than heal and it reduces penance which is a good efficient heal as we disc priests all knows. It makes sure i have my 4piece bonus up again before it runs out.
When the OH Shit comes, i got wings and helping tanks back up.
Rarely i see my smites goes to a melee, there fights that gives smite more healing as well.
Omnotron > Poison cloud and Arcanotrons puddle.
Valiona > Engulfic Magic
Halfus > debuff on halfus
Cho Gall 2 healing, im on the add tank We have extra dps staying on halfus cus i dps the big add as well + healing the add tank.
Theres more to Atonement than classify it as raid healing as smiting is imo more tank healing if u have competent melee avoiding bad stuff. Disc has more routes to go with for same roles, its more a gear choice that defines u as raid or tank healing.
Both ur specs can do both roles as efficiently.
Its just a matter of liking smite or not.