My question about how you might tell if one light source for scanning colour negative might be better than another was rhetorical really, though I really would like to know if there is a way. In the thread that I linked to @Richard1Karash has attempted to do this using is own personal test scene, starting here:
He has also more recently presented the results for the Cinestill CS-Lite and each of its different colour temperatures, if I can find it I’ll put in a link. Also very recently there is this other post where the very poor results were cured by replacing the cheap panel with a Kaiser Slimlite Plano:
So it seems it is possible to demonstrate the poor results obtainable from poor panels (provided you have a better one to compare it to) but I’m not sure that it’s quite so straightforward to distinguish between results from the better ones, particularly on colour negative of course. I don’t know how you do it really, even the manufacturers don’t seem to attempt to demonstrate the difference between their 95CRI and 99CRI offerings.