Mantle of Channeling

Is this damage increase additive? Is that why it’s not used for GOD?

It’s multiplicative. But if you use it you’ll have to give up something else.

As stated, you’d have to give up something else. You can’t use in the cube since Depth Diggers takes that slot and having it equipped on character means using RoRG in the cube. In the latter case you’d be better using the Aughild set than Mantle of Channeling for bigger benefit.

In short, it’s just not worth it.