Maybe this helps. I don't know the quantitative analysis of data from FoCal. It spews out pdfs with images of the target on them plus a score. Here are crops from the pdfs plus scores from tests I did to find the sharpest aperture for the lenses on my 5DIII, all at the same distance. I have pasted in the image at the widest aperture and the sharpest (it is the same for the 600mm at f/5.6). Although the Focal score for the 300mm + 2xTC is lower than for the 300mm bare and + 1.4xTC, it gives the best image of small far away subjects because of its greater length.
Perhaps Neuro could comment?