Correction, I have written 1600 ISO but meant 16000 ISO, sorry.
Hi,
You always loose dynamic range when increasing ISO. But, you may have plenty of DR, so it may not matter.
What really happens when you increase ISO is that the image gets noisier. Why it gets noisier? Because it captures less photons. If you have a sensor twice the size, it will capture twice the number of photons at a given exposure.
So, if you have a 54x40 mm sensor, it would have an area that is 54x40/(24x36) -> 2.5X times the area of an 24x36 mm sensor. So, you would be able to use 2.5X higher ISO settings on 54x40 mm compared to 24x36 mm.
So, if 6400 ISO works with 24x36 mm, you would be able to use 1600 16000 ISO on 54x36 mm and get similar results. That applies as long as the underlying technology is similar. We are not so far from quantum limits now, so great leaps in performance are not really possible without radically new technology.
I would be cautious about using very high ISOs. I have shot ISO 6400 on my A7rII, in good light. recently and found that I got workable results. But, it is always better to keep ISO low.Photons are building the image and you want to capture all photons you can get.
Modern sensors are pretty much ISO-less, meaning that you get essentially the same result if you rise ISO five steps as you would get keeping ISO at base and underexpose five stops and adjust in LR/Photoshop/C1.
Some modern sensors have "dual gain conversion", a neat trick to get a cleaner signal at some certain ISO. Some of Sony's 24x36 MP sensors have this feature, kicking in at 640 ISO. That means that you get better results setting say 800 ISO than using 100 ISO and underexposing three stops. But, The H6D100C does not have that feature.
Best regards
Erik