I've been the first weekend in August twice and while it has been pretty hot the crowds were manageable for us. Most of the rides outside of MK are airconditioned as are the restaurants so we would use our fast passes for the big ticket items rather than wait in the heat. We planned ahead and made our dining arrangements and kept an eye on early entry etc. I have been to WDW many times and while this time of year has been more crowded than most I would say that it was easier to get around than the year I went a week before Halloween
Now with EMH in effect you will have a wonderful opportunity to do things in the evening when it is a bit cooler - don't worry, you will have a BLAST!