Is there a 'sweet spot' where you get the best image from the sensor(?)
My guess would be Yes, there will be a best crop for UHD footage and for HD footage, but they may not be the same sensor crop.
For example, if we assume that the full sensor is 3840 x 2160 then theoretically the best image for UHD would be using the full sensor (a 1-to-1 pixel-to-image ratio). But shooting HD footage on the full sensor may have digital artifacts or moire depending on the sampling/downscaling. The HD sweet spot on this supposed sensor might be at a half crop, again at a 1-to-1 pixel to image ratio.
But according to my calculations (super-35 size, 13.5MP) the sensor would be 4243 x 3182. So again it depends on how the sensor is sampled.
Perhaps someone close to Jim Martin's store would be willing to borrow the camera for a day to run some tests?
I would think the laws of physics would require at least a theoretical fall-off in quality with variable scan mapping footage.
Yes, in theory, I agree. But there could also be several sweet spots of cropped sensor size with similar image qualities depending on if you're shooting UHD or HD footage.