I think the heat depends on what you're used to. I had a job that required me to be outside 90% of the summer, running around after kids and playing sports (in CT- for reference), so when I went to Disney two years in a row for Labor Day- it didn't seem too bad to me- more of what I was used to. The two years I went I only really remember one "rainy" day, and it had only an hour of real downpour - when we were at dinner. The parks cleared out and it was great for touring the rest of the night. I agree, rope drop! You'll be able to get most anything done in the AM if you rope drop. AK is the park I had the toughest time with...Just felt so much hotter than all the other parks. The crowds are very manageable though. I wish we were able to go this year at the same time but I know that will be our "usual" vacation time until we have school aged kids.
I will say, the first year I bought the cooling frog togs and I barely used it. Just bring bottled water.