Busy is relative. Yes, Labor Day weekend is busy...but not crazy busy like Christmas/New Years. The half marathon and the 5k are that weekend. This alters park hours a bit and there were a lot of people in marathon shirts wandering around.
I can't help with wait times. I use ridemax and take advantage of single rider and FP, so I really didn't wait in many lines when I was there over Labor Day. Crowds dropped way down on Tuesday. At DL, only Indy and Splash offered FP (they just weren't needed elsewhere).
For what it's worth, it's my favorite time to go. I like straddling the weekend to take advantage of the full entertainment schedule and to hit the attractions that are going down for holiday overlays...and then I stick around after the crowds head out and enjoy a couple of quiet days in the park. I've done it the last two years, and I'm headed back this September!