The MK restaurants operate a little differently at opening than other restaurants on property. Basically, they want to get people in and seated and processed through their meal. So when the desk first opens, my experience has been that whether your reservation is for 805, 810, 815, or 820, it doesn't much matter - who's ever in line gets seated more or less in the order they get to the desk.
Now does this extend to 825 and even 830 reservations, I don't know. But the short answer is that during that hectic opening time at CP and CRT, they're a lot less concerned about the precise time on your reservation. Do your best to be there on time, but don't panic if you're a few minutes behind schedule.