It depends on weapon itself, since kinds of weapon deals different dmge. U can easily compare normal legend chkram and bow to see how different it is.
In terms of mythics, im not sure if it really adds dps on the equip. but if it do add then its so difficult to check since mythics got its own uniqueness.
It would be better if you will only equip one +2 multi attack ant not ATTACK CHANCE and another +2 multi attack.,., cause in the arena, only +2 is possible.,., cause I tried equiping +4 multi attack., but only 2 is coming out of my weapon.,., so, just use +4 multi attack at compain only not in arena.,., ^^