I've got an '03/NV-5600 & have asked the same question of some of the TDR advertising transmission shops (Blumenthal's, Standard).
I don't haul real heavy (7500lbs). But, I do tow in the Western mountain states, quite a bit. That was the reason for my concerns, initially.
Their opinion was that this trans is very heavy-duty with a lot of lube, in it. At my level of towing, they didn't think extra cooling was necessary. If you are towing extreme weights in the mountains, all the time, this might be something to consider.
What I did, instead of extra cooling, was to install one of the trans. filters. That seems like a good idea, to me. Their was a thread, either here or the DTR, that showed pictures of the aftermarket filter and what it looked like after some use. It was filthy. Trapped a lot of debris!!!. I was surprised at how much "junk" was created in this trans. After seeing the picture, I'm glad I chose that option over the added cooling.
If you use a synthetic lube, the extra heat created by the occasional long uphill pull that most of us will encounter, will probably & hopefully, be absorbed & handled by the fluid.
Joe F. (Buffalo)