No thermo valve on my 2013 and yes I have looked. Engine coolant temps are holding 210ºF with no variance. The trans temp worries me most as it will regularly get 15-20º hotter here by the depth of summer so if the 90-100º trend follows I'll be well into the hot end of DEX6 and the worst of the symptoms.
Best I know the radiator is original as it only has 117k miles (yes, it nuked the trans under prior ownership at less than 100k miles). A 40k cooler is in the cards for when I'm done rebuilding my friend's Nissan and can get my commuter car back and sideline the Yukon. The question becomes how deep does the hole go with rad, VB, TECHM, etc... to get this thing up to scratch for it's job or do I put an OEM HD cooler on it and trade this in for a 2007 2500 and skip the 6spd mess with a good old 4L80E.
The 6L80s are fine. Their issues come from the factory JMBX torque converter, which had a poor design. The back plate would warp over time, and the "hot spots" would shed their friction material into the fluid. This material would block critical ports, orifices, galleys, etc...
They aren't magically going to fail at 200F.
Switching to a billet-backed torque coverter would be cheap & smart insurance. Adding capacity to the fluid would also help, and there are external AT filter kits that are easy to adapt inline with the existing external/aux. coolers.
Understanding the underlying issue, first, then addressing it... that's how you solve a problem. For example: What is the max operation temperature of Dexron VI? It's wayyy more than 200F.
