Cultist’s Fervor is supposed to increase the Mana Efficiency of Bone Curse, and it does for the base ability, but it doesn’t impact the mana drain on the Signet of Agony node, at least not to the point that it states.
I have done several tests with a timer on a level 15 character with 118 mana. The aura from Signet of Agony lasts ~14.9 seconds before I reach 0 mana. When I put two points into Cultist’s Fervor I should gain 20% Mana Efficiency, however, the duration only increases to ~15.2 seconds, so roughly 0.3 seconds longer.
That is clearly not a 20% increase in Mana Efficiency.
I believe what happens is that the increase in Mana Efficiency only affects the activation cost of the base skill and not the Signet of Agony node at all. This is why I get just a tiny longer duration.
If this is the intended way Cultist’s Fervor is supposed to work, then it should state on its tooltip that Signet of Agony is not affected by it. I doubt however that is the intention here since Signet of Agony doesn’t give a new ability; it merely converts Bone Curse into an aura.