Well I do know at least on the Boards it's been mentioned for a while that during the Christmas parties MK's official close time was reflecting 6pm on the park hours on their website. Now that doesn't say much for the average guest but we don't really any much idea just how many people stay beyond (or at least much beyond) official park close time. I can imagine a bit of an overlap maybe moreso from 6-6:30 (just guessing).
Though yeah it bites for the guest to only have 9-6 (at least currently 9am opening). I will say Universal has the same number of hours as MK now as they do for their
HHN. One of the days we are going there the Studios closes at 5pm with an 8am opening due to HHN that night. Now we have park-to-park tickets so currently the plan is to hop on over to IOA til their closing time which is 7pm with an opening of 9am.
The bigger difference, and perhaps a better way of doing it in regards to crowd overlap, is with Universal and HHN they ask daytime guests to leave just before closing time (which is 5pm) and they hold back HHN guests from entering the park/milling about the park whereas at Disney there's that overlap of day guests and party guests inside the park free to roam around.