I'll add that the proper way to monitor the ATF temp on a 5EAT is via the internal trans temp sensors using the OBD port. That gives accurate, in-situ ATF fluid temps from the bowels of the beast. An external gauge will only be relative to the real deal in the trans, showing you that your ATF is being nicely cooled, or not, by whatever cooling setup you have. But once that fluid is back in the pan, it's coming up to cooking temps nice and quick. Sous vide, anyone? If you want an external gauge, then it should be plumbed in before the cooler, as close to the transmission as possible to more accurately monitor the actual ATF temps within the trans.
What's important in a cooler is that it cool an adequate amount when necessary. What's an adequate amount? As David illustrated, it depends on the car, the driver, and the conditions. I've got lots of lady friends with OBXTs that will never need more than stock cooling. Preventing long term fluid overheating is what's paramount. It really doesn't take too many incidents of overheating even quality ATF to severely degrade it. Any stock 5EAT with decent power in front of it is capable of toasting its fluid, even with as much cooling as you can throw at it. Looking at scorched blue transmission steels and clutch packs reveals that's the most dangerous heat source from the ATF's perspective: extreme pressure, sheering, and heat generating friction, with the fluid laid out nice and thin and at its most vulnerable. Built 5EATs and those with VB mods will generate less heat internally than a stock 5EAT. In fact, a VB upgrade is probably the best "cooling" solution for a 5EAT, but I don't think you can put too big a cooler in place, especially if you use a thermostat with it. My testing shows that even with a large Hayden 678 as the only cooler, temps can still linger over 200* for a prolonged period of time in summer heat after having beat on the car.