We always eat early (5:30 - 6), order all the courses and always have time to catch any of the shows. Even the ones that are presented at 7 PM (which are intended for the late dining people) are possible to make if eating early & get a seat since there not usually filled to capacity.
What you can do is forgo the dessert when dining early, go to the show and then return to the DR later on for dessert. The DR will be practically empty & you can walk right in up until 10 PM.
Of course if you like to have a drink at 6 PM & then arrive at the DR door for dinner, you'll probably be given a beeper & called back later during the first performance & will probably be late for a seat for the second show.