If you're getting these gloves, it is likely you already have 3-piece Transcendence (I don't know any guilds that can kill Huhuran and not clear BWL). Also, next patch I doubt ANY healing specced priest will not have meditation, so it's reasonable to assume 30% of mana regen in combat.

In that case:

(Assuming you spend 50% of a fight in 5-second rule, which is a rough average of my own playing, but is a reasonable assumption for most priests. Your assumption of being 100% of the time in 5-second rule is terribly unfair).

15 spirit = (5/8)(0.5*0.3*15 + 0.5*15) = 6.1 mana/5

Prophecy gloves have an additional 6 mana /5 raising their total value to a priest in the previously described scenario to 12.1 mana/5.

Under this specific scenario, the comparison becomes:

+3 Stamina
+2 Intellect
- 7 FR
+8 healing
-2.1 mana/5

Your choice from there. Prophecy gloves can potentially grant you the 1.4 sec FH, which some priests may like to keep. If the priest has spiritual guidance, it's only +5 healing to be gained. If you compare the difficulty of getting loot from Huhuran to that of Gehennas, I still think Gloves of the Messiah are underpowered.