When we were there in March, the CM at Screamin' explained to us that they do not close the rides at a set time every night. Rather, they need everyone in line to get on the ride and be done by a certain time before WOC starts. He said the CM for each ride needs to look at the line and judge when to shut it down. So for instance, when we were there, it wasn't very busy, so he looked at the line while we were talking and said based on the size of the line, he would probably close it around 7:40 p.m. This would enable everyone in line at 7:40 to ride by 8 p.m. WOC was scheduled for 8:15 that night. He told us that if there is a long line, he would probably close it closer to 7:20.
With an 8:30 show, I do not know if that means the CM would try have everyone finish riding by the 8:00 park closing time, or by 8:15ish for the WOC time. I have read that if you are in line for a ride when the park closes, you still get to ride, so it may be that for a later WOC, they will close the line later, not sure on that, though.
HTH!