Service will NOT suffer (well, except for one 'whiny' CM who was upset in front of the Guests - and not acting - about having to work late

). In fact, that incident was a couple of years ago during
Free Dining. The park closed at 8, but my reservation was for 8:25.
I checked this about six times between when I made it and when I arrived. I mean, MGM (at the time) was technically still open because Fantasmic was running, and naturally many shops were, but none of the attractions. But I didn't want to get there and find out it had closed at 8 or something.
Nope, everything went fine. Nice, friendly, relaxing (after other CMs took over for 'that' one) meal, server who was into the role, reasonable food, and a quiet exit from the park about an hour, hour and a half later.
In fact, MANY of my dining reservations have been for very close to park closing time. Often, in Epcot, I get/make reservations for 8:45, 8:50... works out great.