I think we might have had a 8:05, but it might have been 8:10. Either way, you'll still be let into the park at the same time so I wouldn't worry about trying for an earlier ADR.
8a is the earliest, but I find that with the line to check in at the window, 8/805/810 are all pretty similar in terms of how quickly you get your food, photos and get out.
You can get 8am but it doesn't matter just stick with 8:10 or whatever you can get. Get to the park around 7:30an and confirm your ressie with the CM at the gate. Get in line and you will be let into the park early. Once in go to CP and check in and you will be seated promptly. It is more first come first serve at that time so the sooner you go to CP to check in the sooner you will be seated.
We were there this past Saturday. Love that breakfast. Got great pre-open photos, met all the CP characters, loads of pooh puffs and made Tomorrowland before park opened.