I would never go in July or August.
September is pushing it on heat level for me. Give me January any time!
A little local intel here. September is usually worse than July in Florida. August and September are usually the worst months of summer. It's often not until mid-October that the temps begin to go down a bit.
Summer in Central or South Florida is hot, humid, rainy, and sometimes buggy depending on where you are. On the bright side, it's that way in most places in summer, except where it's hot and dry. If you want
real misery, go to Washington, DC or Atlanta in July, August, or September! The difference is, at WDW,
you are at Disney World!
If you go during the worst part of the summer, the prudent visitor will modify their schedule a bit. You go earlier in the day, plan for the daily late afternoon thunderstorms, and take advantage of late EMH. You stay out of the sun as much as possible, wear sunblock and a hat, drink a lot of water, duck into air conditioned attractions during the hotter periods...common sense stuff. And you have a great time.