I responded to him via private message. In case anyone else has the same question:
The Mana Shield Damage Reduction reforge no longer stacks with other sources of efficiency. The higher of the two apply.
A = (Magic Attack * .004) + Mana Shield Rank Efficiency
B = Magic Attack Damage Reduction
Mana Shield Rank Efficiency is capped at 1.8. Magic Attack Damage Reduction is capped at 20 level (or 4.0).
500 magic attack grants 2.0 efficiency, which would set you at just barely below the efficiency afforded through mana shield damage reduction 20 level. If you plan on using these gloves with a magic weapon, the reforge is obsolete. Otherwise, it's a viable alternative to standard defense/protection at best.
Ironically, the reforge would help quite a bit in the cave of trials.