I'm sure disc priest already know this, but I'm shadow only and when I do see disc priest, it's apparent their focus is on healing over DPSing which is how it should be anyways.
However, found the mechanics of disc DPS different and interesting. I decided to mess around in a Heroic and I was pleasantly surprised at 35-50k on recount. Consistently 40k on single target dummy self buffed. I'm a 483 SP but also brought a 496ish SP friend with me and he pulled around 60k-70k using disc spec. Now I queue in as a healer and just DPS and let atonement do the work for me, usually 85% of healing, pre-pull PoM, and Halo/Cascade does the rest. Granted, not a viable DPS spec for raiding, that's why I'd only do it in heroics or be sneaky in LFR. I found mana tough to manage even with mindbender and evangelism but it it was good change using hymn of hope at the right times during boss fights (never use it as shadow) albeit, I'm not spirit geared like a healer. Merely doing this for that wishful thinking that priests would have a holy-based dps spec similar to how they split bear/kitty. This'll do for now. It feels good topping dps meter as a healer, ofc it's heroics and I understand people are usually undergeared, let me bask in my thunder when the DPS whisper "WTF?".
I found it to be an interesting priority rotation. Try it out and add input. I do suggest being geared so you don't wipe group and of course if you're queued as a healer, obviously heal when things get out of hand.
Glyphs: Holy Fire, Smite, Penance
Rotation I use:
*Mindbender on CD
*Power Infusion on CD
-apply and maintain SW:P
-apply and maintain Holy Fire
-Smite when HF debuff is up
-Penance filler when HF debuff is off and HF is on CD
-Halo/Cascade filler single target when HF debuff is off and HF is on CD
*Keep Evangelism at 5 stacks.
*Halo/Cascade on CD when multiple targets.
*Refresh SW:P when HF debuff is off to not waste GCD as smite is a priority during the debuff.
*I cast PW:S on tank prior to pull for haste buff.
*Glyphed Penance when on the move.
*Archangel as you see fit as you don't want to wipe the group.