My friend, did you EVER come to the right spot to get that answer!! LMBO!!
I've owned my 5740C since Feb 05, and 05 and 06 were extreme drought and extreme heat years for us here in Texas. In fact, we're going back to that extreme heat and humidity conditions right now, with our air temps going to hit 101-102 this weekend. That puts our heat index, or what it really feels like outside up to around 110.
I should mention that I am also VERY sensitive to the heat, courtesy of my time with Uncle Sam many years ago!!
I have NEVER gotten hot in the cab while operating my 5740C in the middle of an afternoon like we're going to have today, or have had in the past. My A/C on my 5740C has been a lifesaver, day after hot and humid day, if I'm out working in the 5740C in the heat. Without it, I couldn't have done that work at that time of day. Believe me, I judge any vehicle on the quality of the A/C in these Texas summers and I'll hold the 5740C's A/C up to anyone else's. Never have had any problems, except that "mouse in the roof fan" problem and that was the mouse's fault and not my A/C system's fault! I went to the 5740C from a non cabbed tractor specifically for the A/C and I'm VERY pleased!
I came back in to add to this post that my neighbor across the County Road from me, bought his 5740C after using mine. He too is very pleased with the tractor and the A/C's performance in our hot Texas Summer!
Hope this helps.