While I don't know too much about the effective values of Int vs Spirit, I can say that the correct usage of spells goes a VERY far way- In the many Stone Guard encounters I have been doing in raiding group 2 of my guild, the other 2 healers were going oom before the boss was under 100 mil, whereas I was sitting around 200k mana.. I suggested to them that they start triaging rather then rushing to top everyone off, and use their Nourish equivalent more rather then their high mana per cast time spells, and suddenly, while ppl were sitting at 50% for the majority of the fight, we got to 60% and the healers had higher mana (Around that time, the off tank messed up the taunt rotation, again, and we wiped due to 2 out of phase overloads, rather then the other 2 healers going oom and being unable to heal)
Im pretty certain that mana will no longer be a problem for future attempts. I do have to say this, however- Intellect or Spirit, if you are using the wrong spells at the wrong time, you may as well gem pure expertise for all the healing throughput itll give you.