If at all possible, I would make the ADR for 8am when the park opens at 9am. We had an ADR for 8:05, we got to take pictures in an empty park and we were done (seeing all characters, eating, and a bathroom trip) by about 9:10. So we were only a few minutes behind the rope drop crowd. This allowed for the best park touring. (Those first two hours, we just walked on basically any ride)
Personally, I wouldn't do an 8am ADR if the park opens at 8am...yes you might get in there to take a few pictures, but you would miss out on a great time to tour the park. FWIW, I would plan all our breakfast meals to either be about an hour before park opening OR more of a brunch time (for instance, AK had am EMH so we arrived for that then had Tusker House at 10:30).