To be specific, I am looking at Monday, May 8, which will have Spectro (9PM) and Wishes (10PM) that night. If one skips both of these, how much does it suck down the lines on the rides? Say after 8:30?
I don't have any information for your specific date, but I have found that it helps quite a bit. It doesn't completely get rid of the lines, but it helps. We love PhilharMagic, and we can usually see it two or three times during the parade.
One night during our May vacation a couple of years age, we rode splash mountain 5-6 times in a row with no wait at all. It was great! We even got to see some of the fireworks from the top of the ride!
This is one reason we tend to miss the parades - for us - August/April/October visitors - it has almost always been the case that lines are short or non-existent. DS cannot do lines and we had never gotten at GAC til this past trip - used it twice in AK - so it works for all of us. And as mentioned, Fantasyland and Frontierland are pretty well empty, and you STILL get to see Wishes from wherever you are!!
last week rode big thunder mountain railroad five times in a row during spectro parade. Four of those times never even got off the ride, we were in the front two rows of the train (there were four of us together). As long as there is no one waiting, the cast members won't make you get off the train!