HI there,
Also, lets be clear that the FLUID you use will change your requirements for cooling as well.
If you have not changed from the older Dexron 3 that your transmission was originally equipped with, then your possible need for a cooler will be different from using new formulation Dexron 6.
Remember, oxidation and thermal stress is the largest killer of transmission fluid. 40% of the internal heat is carried away from an automatic transmission by the fluid. It also lubricates and CLEANS your internals of your transmisson.
Dexron 6 will not start to oxidize until over 300 degrees F.
If your transmission ever got over 280, you would receive a "transmission over temp' warning.
Even tracked cars with 4L65 only get to 230 if the cooling system is working correctly.
I would not worry, nor would I recommend an aftermarket cooler unless your temperature gets above 250.
Allthebest, Paul