I will share my experience with DEX VI. which is recommended by the builder of my trans.
While I will agree that it may hold up better in high heat applications there is a draw back if you reach trans temps over 200* that may or may not be associated with other types of fluid.
When I went to pick up my car with a fresh trans and torque converter installed I took it for a shake down run and flogged it pretty good.
With the builder in the car on this trip we noticed that after a few hard runs it would start to shift higher and higher as the heat built up/ all runs were with the ac on.
We went back to the shop and installed a trans temp gauge in the dipstick tube and took it for another ride. When we reached 195* it would shift out 100rpm later, and 200/205* it was 200rpm higher etc. .
This was with a GBP rad and dual fan setup that I was told would keep coolant temps below 180* and therefore trans temps should have been within 10/15* of coolant temps.
Well needless to say the fluid changed color [got darker] but didn't smell burnt.
I have since put the stock radiator back in [same dual fan setup] and plumbed an external trans cooler [stacked plate] with a 10" fan that comes on with the cooling fans and have never gone above 190* coolant temps with ac on sitting in traffic/stop and go traffic, and it stays under 170* at all times with ac off.
I now have consistent shifts every time and the fluid looks and smells new.