The Circle of Mana III effect appears to not work. I tried it on myself (Conviction) and it made no difference, but the description says on companions so I had a guildie try it (Wunshi) and same result, no reduction in mana.
According to mqemulator ...
Slot 1: Decrease Spell Mana Cost by 12%
Slot 2: Limit: Max Level(75)
Slot 3: Limit: Spell(Complete Healing excluded)
Slot 4: Limit: Combat Skills Not Allowed
Bummer too, would have been great to have for the healers hiding with me from AE's.