I've never really seen the point in the mp5 flask for healers. Over the course of 5min it gives you back 2280 mana which just don't offset the extra healing you can get from 125 spellpower. I use the spellpower flask exclusive on my healer
I'm a new raiding priest and my gear reflects that. I'm pumping out the HPS on fights like Sapp and KT, but both I went OOM in the last 25% or so. Some of this is learning the dance out of 5SR better, the other is my aggresive healing style.
For the raid tonight I'm opting for mp5 flask from Patchwerk onwards, and mp5 food, so I can avoid that no-mana zone. There is value, most of my heals hit for a little overheal and get me serendipity, so flask of the wyrm may be wasted overall.
I don't recommend pure mojo for healers at all. When you count the mp5 you get, its practically useless -.- If you want a flask for regen, distilled wisdom is better (it offers some extra mana, regen, crit and even scales with talents o.O), or Frost Wyrm for the spellpower.
Or if you are on a farm raid or have gold to spare a Guru/spellpower+Draenic wisdom elixir combo is better for holy priests and resto druids, and Guru/spellpower+Mighty thoughts for disc/paladin/shaman.