Alright.
I had about 5.6k gs (though I don't think it really matters) and I had about 33kmp unbuffed.
Got up to 37-38k MP on 25mans. And that's with Mp5/INT gems on blue sockets, and SP/INT on red ones.
If you spam FoL non-stop, then you'll get like 80% overheals and your MP will go down fast.
But you still can't run OOM, unless you're actually trying to. It's just impossible.
Say you have 30k MP. You'd need to do 100 FoL to go OOM if you had 0 regen. But you also have Illumination and Divine Illumination.
+ The replenishment from other classes, + mana pot + arcane torrent (if you're horde), + Innervate (if someone offers it).
You're saying: Spam - spam - spam flash of light and holy shock.
And nothing goes to waste?
What about the priests's AoE heals? The druid's HoTs? the shaman's chain heals?
When the tank healer, heals the raid, then the raid healer's heals are over-heals, and the pala goes OOM faster.
On the other hand, when the tank healer, heals the tanks, then the raid healers focus on the raid, and everyone's job becomes easier.
If you're doing TOC, and your tank is in full ICC gear, then yeah, he won't take much damage.
But when you're doing ICC25, you can't expect the tank to take little damage. The fights are mostly about the tanks getting a$$-r@ped, and they can't be healed easily if you use flashes of light.
Well, then what happens when you actually run OOM? Or when the boss gets a +90% dmg and +50% atk. speed? (/point @ Festergut or similar bosses)
You've got to know when the tank will take much dmg, and be prepared for it.
You can't expect to do a crit every time you heal.
Say that for some reason, the tank goes down to 5k HP. The first thing you'll do is Holy Shock + Flash of Light.
if you're lucky and get some crits, then you'll do (approx) 15k heal, in 2 seconds ( that's 2 global cooldowns).
If you're not lucky and you don't get the crits, you'll do about 10k heal in 2 seconds.
BUT, if you've got more haste, then you can just use a Holy Light, and heal a nice 13k(non-crit) in 1.3sec and then an instant Holy Shock for 5-7k more. With criticals, then it'd be about 20k + 5-10k in 1.5sec :)
You said you go OOM pretty fast when you spam heals.
Also, divine plea is (in my opinion) useless.
You can't rely on it. If the tank healer, heals for half the amount for 15 seconds, then the tank's screwed.
I only use Divine Plea on situations when I know that I won't have to heal for some seconds, just to get some spare 2-3k MP back.
Hope I didn't bore ya :P