is there a notible difference in those mana overflow skills beside of that spell attack bonus? like is the chance of dealing an aoe damage higher on 2 in comparison to 1?
The chance of trigger depends on the number of targets around you. If one target, less chance to zap, if more, you will see zap come out more often.
After all, justice in this world is just a bunch of principles, made by those with power to suit themselves.