Just to clarify after testing Decoy does NOT affect the actual damage output of Hellfire. Surviving the attack on 100hp isn't a feat as a MNK or other class, decoy won't go anywhere to helping that figure, even if you get the feeling it does.

Secondly, you are smarter to use Fists of Earth over Fists of Wind in the Ifrit fight as it is his massive elemental weakness and you gain boosted defence which will actually do something to help your survivability.

Feel free to throw it on and see your damage output sky-rocket

With all this said though, when you factor in worthwhile skills etc, it's actually just smarter to PGL it and have sentinel, self-cures for emergency purposes during jump phases etc and general survivability over the MNK.