I guess it depends on what you're doing, but I'm hardly EVER even close to oom on my disc priest because of Rapture. If you're raid bubbling - even just to keep Borrowed Time up (which you should be) a big boss AoE can burst multiple bubbles at the same time, returning half your mana pool all at once.

Now, naturally I'm just starting to raid (ToC10 for gear lol) on my disc priest, but I never found myself running low (ever) because of Rapture. And though I've never raided it on my disc priest, there's so much continual AoE going out in ICC that there should be no reason Rapture wouldn't be returning mana quite frequently (I think there's an internal CD, but even with that every time it wasn't on CD or two or more shields popped at the same time, you'd get a big return).

So, I don't really see the point of that huge mana pool. You only need as much mana/regen as is required for the fight, and no more, in my opinion. On my druid (my main), I stacked haste and spirit for a long time, trying to get the (very high) haste cap while also having enough mana to last the whole fight. With better gear, I've found a good balance, so now I've started replacing some of those haste/spirit gems with haste/spellpower or just spellpower. I can't see you burning through a 41K mana pool with Shadowfiend and Hymn in ANY fight, even if you were casting straight from start to finish, so you would probably be better off getting rid of some of your wasted mana pool for more throughput. Look at your mana pool at the end of a casting-intensive fight. Do you still have 10%/20%/30% of your mana left? That's what you can get rid of. It's like overhealing - overhealing is wasted mana, but mana left over at the end of a fight is, imo wasted mana too. If your regen is too high or your mana pool is too big to get rid of, the stats that are increasing that could have been used elsewhere to make that shield absorb a little more, or that penance heal a little more or whatever.

But that's just my two cents, if it's working for you, it's your character =).