First I agree with the idea of this spell only going to RDM only. I think in this game, it just seems right for them to have.
Second, I think an AoE version should be made exclusive to SMN, say in the form of a Carbuncle blood pact (I believe he's had that in past FF games).
Finally, make duration and max spells reflected tied to enhancing magic if you're going to do it.
And before the inevitable is mentioned, I'm fully aware I don't have the job leveled, but I've been around enough great mages in my 7+ years of playing this game and playing every other FF game up until this one many times through to have a good enough understanding to make a comment.