If the problem is that the gnome's access to several mana potions makes the boon too powerful, how about changing the cost from mana potions to something else?
I think it wouldn't be too far from JJ's theme, and it would also be much fairer overall, to have it cost something like a glyph rather than a mana potion. Of course, the piety cost/mp bonus would have to be changed to account for that, and there would be a need for some way to choose which glyph to sacrifice (unless it was chosen at random, which is even more fitting for JJ, but way too steep)