ISO Does Not Matter (Sort Of)
In this test, aperture value and shutter speed are kept the same, the only variable is the ISO value.
The camera used was Sony A7IV but the result should apply to any camera with a linear ISO progression and dual base ISO.
The picture was correctly exposed at ISO 6400 and pushing ISO 1600 & 400 to the brightness of ISO 6400 shows minimum effect on image quality and noise performance, however, pushing ISO 200 to the same brightness resulted in an unusable image.
ISO 6400
ISO 1600 +2 stops
ISO 400 +4 stops
ISO 200 +5 stops
Pushing both ISO 200 & 100 to the same brightness also shows the same image quality and noise performance. It is very likely that A7IV has a second base ISO at 400, hence the cleaning up at ISO 400.
ISO 200 +5 stops
ISO 100 +6 stops
Pulling ISO 200, 400 & 800 to ISO 100 which was correctly exposed shows the same conclusion. Highlights start breaking at ISO 200 but it was useable, and completely breaks at 400.
ISO 100
ISO 200 -1 stops
ISO 400 -2 stops
ISO 800 -3 stops