With the assumption of a high quality transmission fluid cooler, what is the reason for increasing the fluid capacity through a trans pan for example? If you have a good cooler, doesn't this just make your service intervals more expensive?
Additional oil doesn't help cooling, it's a heat capacitor and will mean that it takes a bit more abuse before the fluid gets too hot but will not improve cooling at all.
I'm struggling to determine any benefits (true benefits) to additional capacity. We used to see this as a common thing to do on high performance builds on old V8 engines because we would move to high volume oil pumps along with increased engine RPM. Those two things combined would suck the factory 5qt oil pan(s) dry during sustained high rpm operation. So adding 2-3qt of engine oil was a real and true solution to a real problem.
I am trying not to think hard, but I added 2.5 quarts of engine oil capacity to an engine I built a while back. Engine had known documented problems with running the pan dry like dyezak mentioned. Another major advantage on this setup was a significant safety margin if I ran low on oil. Basically I could lose 2.5 quarts and still be at factory oil specs.
I'm not thinking right now if the same holds true for the trans, but I don't see why it wouldn't.