BJL,
Have you ever tried simplifying your arguments?
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Given how you so massively misinterpret what I have been trying to say above, perhaps I should. I suggest in particular you re-read my post #111 in this thread, in which I say in part
... I can see the speed advantage of using a larger format and larger focal lengths to achieve good IQ at the combination of high ISO with low f-stops, but see no point if one ends using long, slow lenses, getting combinations like 400mm, f/5.6, ISO 3200.
...
In general, I see little or no high ISO IQ advantage in using a larger format (larger pixels and longer focal lengths) in situations that then involve using telephoto lenses slower than f/2.8, and even more so, slower than f/4, which leads to declining AF performance.
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It should be clear that I am talking about claiming a high ISO advantage for larger formats in situations where that high ISO speed is used in conjunction with a relatively
slow lens (or a lens stopped down to a slow aperture for adequate DOF), meaning
apertures of about f/4 or slower. This applies to all the zoom lenses that you have mentioned using (Sigma 15-30, Canon 24-105 and 100-400) except for the slightly faster f/3.5 at the wide end of the Sigma.
In that situation, the claimed high shutter speed/low light advantage is illusory, as a somewhat smaller format can achieve very similar results at the same shutter speed with a suitable combination of shorter focal length, lower aperture ratio and lower ISO speed. For example changing from 35mm format, f/4 [or f/5.6] ISO 3200 to "APS-C" format f/2.8 [or f/4], ISO 1600.
I have already said that the larger format has a speed advantage when it can use the combination of high ISO and low f-stop. So why do you use low aperture ratios like f/1.4 vs f/2 in your attempted counter-example?
Finally, note that I am not claiming in this thread any image quality advantage for a smaller format, nor denying that larger formats have some IQ advantages; my point is only to dispute _some_ claims of IQ advantages for larger formats, based on the persistent and often false assumption of using equal f-stop and ISO speed with a larger format and greater focal length.
I am avoiding comparisons of imaginary future products, so will simply say this: regardless of how sensor and photosite technology develops, the only way to get any advantage from using a larger format is to use a larger focal length, and that requires either (1) higher f-stops, and thus either higher ISO speed or lower shutter speed, or (2) larger, heavier and probably more expensive front lens elements and less DOF. Option (3) of using equal pixel size and equal focal length and cropping is of course simply a more bulky and expensive way of reproducing what could be done with a smaller format.
