I've gone to WDW on the Sunday before Memorial Day but not going to a park until on Memorial Day. On that day I went to Epcot. There was a long line to get in, but maybe if we'd shown up at 9:00, instead of 9:30, it would have been a lot better. However, once inside the park, Epcot didn't seem crowded at all. Wouldn't have dreamed of trying out MK, however!
As far as heat, I thought it was pretty hot, but then I hate the heat! I wish we could have gone earlier in May, because of the heat, not because of Memorial Day. However, vacation scheduling didn't allow it.