As a disc priest myself, I use PoM on CD during sustained AoE. I then try to spam PoH, unless someone in the raid is low on hp. I often find my PoH way too slow to keep ppl alive that are already low. In that case I prefer casting a penance or flash heal until everyone is above a certain safety margin (which depends on your fellow healers and the encounter ofc). If I had to hazard a guess, I'd say about 50% hp is the safety margin on meg rampages #5 & 6 for me.
If the raid is roughly 80% or higher, I start to consider atonement instead if PoH. I never stop casting PoM for AoE healing though, in fact, it's often the first spell I'll fire off. Also, don't forget PW: Solace since it's a nice smart heal.
In the end though, burst AoE healing is definitely our weak spot. PoH feels weak unless you have full archangel, but is the only spammable spell we have for AoE healing. Luckily, spirit shell makes up for a lot