Exactly.
mia material passes was just an "update" of the mia material x that was broken. No passes.
I don't think that's quite true. I think x_passes is constructed a fair bit differently in it's coding to facilitate the different shading interactions being separated into different passes. mia_material_x being more efficient for single pass renders since it doesn't need to check for pass settings, nor do any extra calculations for each type of interaction.
If x_passes was basically the same at the code level, it would seem quite odd that iRay still doesn't physically support the material, but does support mia_material and mia_material_x.
I think I might do a test to see if it really does have any performance impact..