In my opinion, the stock cooler is sufficient. If you have not upgraded your torque convertor, I recomend it highly. It, alone will make a drastic reduction in temp. transmission heat is mostly generated by the slippage in the convertor. A better TC with a lock up controller and a good valve body will let you tow really heavy with moderate temps. I have the ATS convertor and controller and could not be happier with the results as seen on my XMonitor. My GCVW is just under 20,000, and my max temp has never exceeded 180. It runs hotter in city traffic, when it is un-locked, than it does towing!
Ron