I have a B&M racing trans cooler, the largest one mounted in front of the radiator. Deep trans pan. Is there a difference in heat buildup city/highway? So far I haven't driven far enough to even show anything on the temp gauge. I'm not even sure it's working or else the cooling system is rather effective.
with stop and go driving, if there is gonna be any amount of converter slip....its gonna be introduced more in this type of driving environment. and when you're working the converter...thats where the heat gets generated. Plus it would be assumed that the volume of airflow(when you say city drivng, I figure longer periods of slow speeds) isnt going to be as high, so the cooler probably doesnt radiate quite as much heat as it does with constant 55mph air flowing thru it.
On the highway though...it doesnt take much throttle input to maintain speed. The momentum of the vehicle does most of the work for you. So, you arent really heavily loading the converter at a steady state cruise. Slippage should be minimal at this point, and heat production should be lower as well. not to mention the higher volume of airflow working in your favor.
Even still though, with your good cooling setup, and assuming that converter isnt a ridiculous sloppy mess at slower engine speeds, it shouldnt create a radical amount of heat during city driving either... My old converter was real bad. It only had about 3200 rpm of stall speed total, but it didnt even begin to get tight til about 2700 rpm. it was very loose at 2500 and under, and it clearly made more heat in stop and go driving. I still kept it alive for a long time though!
An 8 inch treemaster stalling almost 5 k isnt exactly a cheap converter by any standards, so it would be worthwhile to add a trans temp gauge in your situation just to be sure you keep them temps well under 200.