Temp Sensor location will give different readings, sensor located in the Pan will read much lower than Sensor in Line out to Cooler, line Out/Feed to Cooler comes directly from the TC, Line In/Return from Cooler goes directly to Lube circuit.
Recommended/Ideal Temps >>
http://www.tciauto.com/tc/trans-life-expectancy
Just like Engine oil you want the Trans fluid warm enough to boil off condensation & provide proper lubricity but, cool enough to prevent damage to Trans & Fluid.
From PATC site:
Transmission Cooling: ( You must have a transmission cooler in your radiator regardless of what someone has told you for the transmission to last. Auxiliary coolers are just that, in addition to. Water cools 32 times better (faster) than air always, period. In the case of air vs. water, there is no contest - water is way better. The transmission fluid comes directly from the torque converter at a much higher temperature than the water in your radiator and is cooled to the water temperature fast. Then it goes to the auxiliary cooler to be cooled far below the water temperature. If you don't need a cooler in your radiator why does GM spend all that money doing so? If you wanted to cool a red hot piece of steel fast would you stick it in water or air, see the point. )
A Strip only car can get by with an External only Cooler as it's not in constant use. It should be a Large one tho'.
Cooler in the Radiator also quickly brings the Fluid temp up to proper operating temp in cool/cold weather.
Todd