We have been twice during free dining at the end of August and this year am trying the 1st week of Sept. I don't find the head too bad since you are in and out of a/c most of the time or in the pool. I like the fact that it is easy to pack because there are no long sleeves or pants required and 1 pair of shoes for the week (crocs!). I will say that AK was VERY hot and you must go there at opening and leave when you can't take the heat anymore!
So I'm probably dreaming!!! But then I am always dreaming when I think of Disney World!
I am realistic enough to know it will be moderately crowded that Sunday and Monday (Labor Day). I'm optimistic that the crowds will thin out considerably after that. If the crowds are gone, we can tolerate the heat!