OK, I see where you got 2.5L. But once again you go to such extremes.
I just gave you this example. It had me taking in just 1.3L of fluid (2 x 23oz bottles). Even if I pushed that to 100g carbs, that’s still just 7.4% w/out any additional water. Although real world testing for myself (n=1) has told me that when I push 100g, I do need to add a little more water. So I’d say 7% is probably a good limit for me on max multi-hour efforts.
Per skratch’s blog that I posted (not sure if you are actually reading):
Depending on one’s hydration state, blood osmolality can range anywhere from 275 to 295 mOsmol per kg of water. Our Sport Hydration Drink Mix has an osmolality of 160 mOsmol per kg of water, primarily because of the lower concentration of carbohydrate
Skratch themselves say they made the concentration lower than that of the blood concentration to create a gradient of passive fluid travel into the blood. If you look at those numbers it’s about half. So they give you room to add other sources of fuel like gels and food without creating too high of a concentration. In my example I had a gel and ended up at a 6% solution, which per your posted study is around the best concentration to be.
The literature that studies the 90g/hr limit is only looking at fueling. Fueling transporters are active 1 way transporters. Except for the little bit of water hitching a ride with the fuel across those active transporters, the main method for fluid transfer is passive and is 2 way. Basically you can’t F up the fuel transfer, but you can F up your fluid transfer.
“Any decrease in absorption and emptying is minimal”. The study you posted, not posted by me, disagrees with that