I have gone in mid August a few times. The weather can actually be quite nice. Better than the weather in August at WDW,(most likely anyway). We had temperatures in the 80s, 60s at night, no rain, both times (2011 and 2013). Of course, anything can happen with the weather. Could go to 100 degrees or something. But probably not humid.
I will be at DL again this August, for the
D23 Convention, which is August 14-16. But, I don't think the Convention has a huge impact on park attendance.
Usually crowds are a bit better before Aug 16 (?), because many local AP holders are blocked until around then. For this reason, we always come in the Sunday before the Convention, for Park time, vs. staying afterwards. I think many locals return to the parks once they are unblocked, before school resumes. (understandable!)
Parks are open much later at DL vs. WDW, which helps tremendously.