My testing shows +15 healing potency (materia) is about the same boost to cures as +21 MND (3 aquamarine rings). Each boost adds about 50 hp more per cura. So with nothing on but the +15 healing potency and +21 MND, my regular cura averages 1150. Interestingly enough, adding an additional +17 MND on top of this does not seem to do anything at all (average cura still 1150.) So maybe there is some kind of cap between +20 MND and +38 MND.

Also interesting, +44 magic critical potency did not seem to affect my critical cures at all (1350 avg for critical cura, no gear.)