Personally never liked a by-pass valve on ATF aux radiators. More so if the ATF is also plumbed into the engine coolant radiator, where it normally has more thermal rejection capacity than an ATF AUX radiator
Let the main, engine, coolant radiator take care of the ATF rejection and in extreme cold ambient, heat it enough to vaporize any H2O that condensed into it
The two main sources of heat generation is in the TC (shearing of the fluid) and at the gear faces sliding against each other. Then the pumping & frictional losses
If designed correctly, the gear lube (in this case ATF) will have enough film strength to NOT be wiped off. Theoretically, there will never a metal to metal, but we live in the real world and there will be metal to metal occasionally or often since this is a towing forum. Plus, all lubricants degrade towards their thermal spec limits.
Meaning the shear and film strength lessens as their spec operating limits are approached or exceeded. Why I'll keep my ATF as cool as possible, but stay within it's spec operating temp range
Just referencing a max temp leaves out much data. What is your ATF temp where it starts to break down enough to create varnish? Smoke? These are not the ultimate spec limit for its thermal rating. That stuff happens below that published highest limit
