Busy weekend, Halloween and all, not much sleep, not much wow time, but I found enough to do a Word of Glory testing regarding Holy Power.
It has been said on very serious sources (like elitist jerks) that Word of Glory healing power is exponential regarding the number of stack of Holy Power you have, hence asking us to only use it with 3 stacks if you don't want to lose serious hps.
So I did two batches of tests, one with a nooby lvl 13 Holy Paladin created for the occasion, and one with my lvl 80 ilevel 264 main. Each character used WoG 5 times with each 1, 2 and 3 HP stack, and I compared the results between each other, and between character to see the influence of the spellpower coefficient part of WoG.
WoG healing of mostly linear in the number of HP stack. Which means that a one HP stack WoG is roughly 1/3 of the healing of a 3 HP stack WoG. It seems that with more than 3K spellpower stacking is slightly more interesting, but at low level you really see WoG being linear in the number of HP stacks.
So you can use WoG with 1 or 2 Holy Power, the healing is not exponential. Though remember using WoG is a use of a global cooldown too, so a slight loss of hps in the end. But considering it is not exponential, you should not refrain from using it in emergencies when moving, even if not at full Holy Power.