You forgot camera angle, focal length, light position, volumetric lighting, radiosity, what type of light (point, area, spot, infinite), proprietary materials parameters and a few dozen things I can think of that a good artist can use. Also, how do you determine what objects to use in the scene? Maybe one program does flesh tones better while another does metallic surfaces better. Mind you, I don't necessarily think the idea is a bad one. I just think it may be harder to create a test that's both fair and conclusive

I'd rather stay in my lane than lay in my stain!