Why would you expect a 'steady' 80C? The ATF heat exchanger lives on the cool side of the radiator. if the temperature at the top of the system is 180F, a delta T of 20F across the core is what you would expect. It would only get hotter if the engine was under heavy load or the fan was off. So for sure it's going to be tough to have the transmission at a steady 80C. If the fan comes on, it will drop right down to about 65C, and that's where I would expect it to stay under most operating conditions.