We went during the last week of August last year. It was very hot, but not too crowded. WWOHP wasn't bad at all, 20 minute wait for FJ, everything else was walk-on, except Olivanders. That had the longest wait, almost an hour! Shops and restaurants in Hogsmeade were packed, as people soaked up the AC.
The rest of the park was rather empty, but we really didn't linger - the girls only wanted HP, and I don't do the big crazy rides, (except FJ!) so we just wander our way around for a while, until I missed the quiet of WDW, and we went back to our WDW resort.
We found that WWOHP empties out later on in the day, as most people rush to go there first thing in the morning.