Thanks for the feedback, but it is unlikely at this point we would change the terminology, as it would break files.
As for the addition of yet another opacity value, which operates on the current one (which is already global to the material), the general way to accomplish this would be to modulate the opacity value or map. However there is yet another way, and that would be to reference the material in a blend or stack material, and apply the second opacity there (this could be done using either the per-material opacity in the blend/stack, or at the global level in the blend/stack). This can be a useful method of working in general, since it allows defining one material, and yet using it slightly differently in different places, by inclusion in different blend/stack materials.
As for how any of this affects performance, provided we are doing our job right, we will not require any specially-privileged extra opacity — the system is very generic (hence the consistent usage of “opacity” in the various places it appears), so regardless how many levels of opacity are applied, if they sum to zero for a given impact, the relevant optimizations will be applied — and if they are not, that would indicate we have coding work to do, not the need for extra parameters.