It says I'm supposed to get 39 demonic fury per fel flame cast. (I'm trying for fun, Demonlogy with GoS...lol) So, the only way this would be viable is if Fel Flame works as it should.
Fel Flame is supposed to give 1 demonic fury per 1000. dmg, (I don't think crits matter) but yea, you can even look at the spell and you'll see the damage matches the fury gained.
According to fel flame it is supposed to deal 39,595 damage and give 39.526 Demonic Fury. Yet per cast it gives a flat 15 fury, no matter what.
So yeah, you guys noticed this? Just try it out.