Well I have just been over there, not staying on site, and we caught an extra evening open hours at Magic Kingdom and it was absolute bedlam. We had only gone in to catch the 10 pm fireworks and the 11 pm parade but we couldn't get out of the park fast enough. Buzz Lightyear had a queue for 1 and a half hours.
What we did was to hit the parks early and not on site early hours days. We got to the parks about 1/2 hour before opening. The Disney ones made you wait till the official 9 am opening but the Universal ones didn't. At Universal we had whizzed around all the rides a few times with no queuing/or hardly any before it started to get busy. The Disney ones were the same, we just hit the park early and got all the rides done before all the late risers hit the parks.
On an evening in Epcot Soarin' had no FPs left and a standby queue time of 2 hours. However, when we went first thing in the morning we were straight on it and got a FP to ride before we left.
Personally I would avoid extra opening hours like the plague and just get to the parks early following The Brit's Guide for quieter days. We got absolutely stacks done first thing.