is not bugged, it wont work with wizard harmony, it was buged previously when you were using, now they just fixed it, qq for mages =(
i read in rochand that empowering echo only supposed ~6% (can't remeber exactly ) more damage so imo double casting is much better for some burst damage in pvp (and stun bonus :d).
yeah its about 10% max, try to hit w/o shots with r grade weap and with s grade weap, its not big difference