My experience is that units start to feel the heat at armor level 5; at 4 and below they seem to do OK. At 5 I have to be very careful with them and while they last a while they end up exhausted. Above 5 and they really suffer.

EA plays the way I do - keep a couple provinces armor free or armor light and use them for desert duty. I also use a lot of unarmored troops that I bribed or bought as mercs in the sandy lands.

ichi