Anyone tried attaching elemental magic potency materia on gear?

Tried comparing +40 fire magic potency vs +0 fire magic potency
It seems that there is no difference in the dmg output when dealing with fire magic.

As it is a stats not shown in the attribute, was wondering if elemental magic potency is
bug or overlook by the developer. I mean, if they had tested it, i'm pretty sure it
shouldnt miss out as the result is pretty obvious (sometimes the +0 even outdmg the +40 fire magic potency) LOL. (Yea, the rest of the gear remain the same throughout my test)