I don't think it is exactly 0, but fairly sure it does decrease enmity gain, and it might just be for spells acquired via addendum. The nice thing though is that SCH has an oft forgotten ability to test this. Run some tests (need about 100 to get a concrete statement) but slap a Libra on it. Both cast the same spell with no gear, libra. Then one casts with the gown and the other a comparable amount of INT and MAB (new mob) then check the enmity levels. It is best to do this with a PLD who has max enmity in order to properly compare enmity levels between the two mages.