I work in engineering and design and support heavy offroad equipment every day. High fluid temperatures (hydraulics, transmissions, & engines) are always at the top of our battle as we constantly have to educate folks who don't know fluids.
If the warning comes on when you hit 260F - it might as well say too late! ANYTHING over 200F is insane. I install aftermarket coolers on my own vehicles where needed to keep fluid temps under 180F and it has proven itself well - over 250k miles on my last truck with original automatic still working fine.
Look into publications such as Lubrication Engineer, Reliable Plant, or Lubes & Greases. Oxidation and permanent damage begin at much lower temperatures than many think, and even though synthetics are great, they won't let your transmission live to 250,000+ while running consistently over 200F.