Two sensors is not overkill. I use a Dakota digital and measure trans temp at output line and again at return line just before going back to trans. I use the radiator cooler than go to a 29,500 btu B&M plate cooler, then to the factory aux cooler and back through a remote filter. There are 180 degree thermostats between all coolers.
The 5R55E trans has an internal temp sensor and does not let fluid flow out until the trans is warmed up. At least that is what I have been told. My temp gauge reads ambient until a few blocks, then jumps up to about 20-25 degrees above ambient. If below freezing, I've never seen anything below 80 except of course on my return line.
This maze of gauges, coolers, thermostats and filters drops trans fluid 50-60 degrees sitting still and 60-80 degrees moving. Hottest output I've ever seen was breifly 214 but the return line was down to 142. This only lasted about 2 seconds. Typically output temps are about 140, up to 160s under extreme conditions. I off-road in the SoCal desert in summer and pull extremely heavy loads (11,500 lb CGVW) to Oregan. The Ex ends up in the High Sierra in winter, hence all the thermostats in line.
I generate the most heat by far when the torque converter is unlocked, regardless of the other conditions or throttle position.
