We're from MA as well. We did August 17-23 in 2013. We did fine. It's much easier to go from Summer in New England temps/humidity to Florida Summer, vs. the one Columbus Day trip we did when Florida was having high 90 temps. Much harder to adjust when the temps are so different from here to there. (That's when we decided if we can survive this trip, we could do a summer trip!) I'd liken Florida August temps to a muggy July heatwave in MA. Do expect the daily afternoon thunderstorms. Crowds weren't too bad in late August, and it's a cheaper pt. season in late August. If you spend a full day at the park, seek out all the air conditioned rides/shows during the heat of the day.
My funny August trip story...we got caught in one of those downpours, and while waiting for a boat to VWL, my oldest said 'We're going to get hypothermia!' No honey, it's still 85 degrees out, you are not going to get hypothermia!