You have a couple of technical errors there Ian
Bright sunlight to dark shadow can be as much as 20,000:1 Far more than 5 stops.
At least in 8 bit files, the dark black end of the scale is only a value of 0-15. As it increases to 14 bit raw, your values can be 0-1024. If you are talking about where the vast majority of the graduations, the shadow has much less than the high end of the scale. I'm not really sure what you mean by 75% of the information is in the dark end of the scale. If the image is 75% dark, then 75% of the information is in the dark end of the scale. Maybe you can phrase this better.
The images that are most demanding are images with the sun in the frame.
Another couple of stops of DR would have helped me out here. Which is why smart folks don't go to 6400 for high contrast images. 100 ISO all the way.