It only makes sense to bypass the radiator cooler if you don't understand how the system works.
Many people, trans shops included, seem to think that the radiator cooler is there to warm the ATF when the trans is cold. THAT IS NOT TRUE!!!!!!!!!!!!! I've tested this as cold as -40F and the radiator cooler ALWAYS cools the ATF. Always. Without exception.
I think that most people miss that the section of the radiator where the trans cooler is located is in the area where the already cooled engine coolant is located. So if the engine is at 200F and the ambient air temperature is 0F, the coolant temperature is going to be near 0F, never anywhere near 200F.
The radiator cooler is a VERY effective cooler. If you bypass it you will lose a lot of cooling capacity.