What is the function of the Opacity property in a material with additive blending mode? Does it do the same thing as multiplying the Emissive Color with the opacity data? Or perhaps does it allow some optimization like indicating which pixels should not be evaluated at all?
There is no optimization from having 0 opacity in additive if that is what you are asking. Those pixels still cost.
I am not 100% sure if its a multiply going on in code but the result is the same as if you had multiplied the emissive by the opacity.