Not sure how i do it, but in ulduar raid 25, cleared till auriya, im 4 th place healer as disc. I mostly not far off from other healers total healing. Mostly 3-5 % lower.
mostly disc priest should throw out shields to lowest targets after or during aoe, before trying to heal. Also penance lowest targets or keep it for the heavy hits on the tanks to get it back up fast.
Also im heavy on haste i got 17% , although our heals are small, due to our speedy heals, a disc priest shouldnt lack to much behind, in some fights can be even near top healed.
i think i managed 1800+ hps, offcourse the more damage that needs to be healed the higher the hps.
disc priest is best to stop damage and pick up forgotten targets back to full hp with penance, basicly we fill the gaps where other healers dont have the time to heal some people in time, the disc priest can quickly react and get the people who need it alive, till the other healer can boost them to full hp.
Most of your healing comes from penance, and so the best u use it, the more healing u will do, also doing shield + penance + flashheal, increases your healing since u can do nice burst healing. Also glyph shield, helps alot since it heals when damage absorbed. But i mostly use to see how much damage i aborsed, since it heals 20% of aborsb, so i muliply x5 and then u have good estimate of absorbed damage just for the shields, and if u add that to your healing, u wont be that far from other healers.
Offcourse a disc priest who just heals the tank wont ever be able to reach high hps, u should mostly do both