I previously installed an aftermarket transmission cooler in a 2006 GMC Envoy (4.2, 4L60E). I installed it "after" the factory oil-water heat exchanger built into the radiator (that is, the hot transmission fluid would first pass through the radiator, then through the aftermarket cooler, then return to the transmission). At the time I installed it, I remember that "everyone" said this was the right way to do it because it provided more cooling capacity.
Thinking about it more I am not so sure. The 4L60E does not have an internal thermostat, therefore it would rely on the radiator to regulate the fluid temperature. Therefore it would rely on the radiator to cool AND heat the transmission fluid, since the radiator water temperature should reliably be around 195 F during normal operation.
By placing the transmission cooler after the radiator, it seems like the transmission fluid might get too cold. At the very least, this seems like it would cause excess fuel consumption, but a cold transmission is also bad for longevity.
Wouldn't it make more sense to place the aftermarket cooler BEFORE the factory radiator, therefore ensuring that the transmission fluid temperature matches the radiator water temperature?