yes the evening extra magic hours do seem to increase park attendance for the entire day.
We tested out that theory only once, going to the AK on a day it had evening EMH and indeed park attendance seemed to be twice what it was on the day we went when there were no EMH of any kind.
other than that one day in AK, we made a point of never going to a park that had morning or evening EMH on that day and found that even in the peak July period, the crowds in the parks were bearable.