We've been in August many times, 7-8 I think now.
Yes it's certainly hot.
From mid August onwards, the crowds lessen, as do the opeing hours often. Although I notice Mk is open till at least 10pm all August this year, good news for us!
We generally find that the only time the parks are 'fun' in August, is in the mornings. Late afternoon and evening can be so busy, that even slower rides have mammoth lines. Because of height restrictions, the scarier rides like Rockin Roller Coaster, often have shorter lines than tamer ones.
Often, fast passes are gone on busy days, sometimes by midday for things like Test track.
That all said, it's still great. Just make sure you're up early everyday to make use of the shorter lines in the morning, and if you're stayinn on site, make good use of early entry and extra magic hours.
See you there!!!
Ian