Personally I never drove faster than 85MPH, because my car was designed for stateside driving. US transmissions don't have coolers like the European. The German wife of a colleague melted the transmission of a brandnew Dodge within six months by driving back and forth to visit her family in a city forty miles away. He said she kept it at 110 (top apeed).
Auxillary transmission coolers are a snap to put on. just bypass the line out from the radiator for the fluid from going straight back to the transmission, go to the cooler, and out from there back to the transmission. Or if you don't want it to be tied to engine temperature at all, just run it directly bypassing the radiator altogether both ways(however running it through the radiator first helps in winter). I have run my 455 powered Cutlass w/ a 2004r overdrive (a transmission that three shops told me should not be behind a big block) regularly up and down the highway, some at 90+ mph, as well as some thrilling 1/4 mile runs, some sanctioned at tracks some well, i plea the fifth, and the only thing ever to go wrong is I had to replace a defective 2nd gear band... But guess what, I still get 26 mpg hwy in the speed range everyone practically drives at...