Bleh, you have all forgotten that you miss out on Holy conc - therefore losing over 16% (I would guesstimate sth about 20% w/ 5 sec rule taken into consideration) of your mana while doing what you say you are about to do: MT healing with gheal (and shield for the disc specc)...
so, good luck finding 20% mana return for your nuke... (and i am consciously not taking serendipity into account right now!)