We were there last summer August 16th to 25th.. we didn't find it too overly hot.. Our second time to WDW, and our 6th time to Florida. Strategies for cool down is sit in front of fan with water spray.. sit inside air conditioned restaurants for a break. Lot of rides you can wait indoors during the hot weather, the rides you wait outdoors can be done at night as well. Take advantage of early morning entries and late night closings. If you're staying on the resort, then you'll have access before opening and after closing on certain parks each day.

I would max out value, and go at least 8 days, that way you aren't in a rush. If you got young kids like we do, its' good to book some extra time to go to the water park, or dip at the hotel pools part of the day. We found that the Disney Dining Plan snacks came in handy for a nice cool down for everyone including Grandma.