We have had 8am ADR's with park opening at 9am several times (at least 3) and have always arrived for breakfast on time. We drive and park. The lane on the far right at the parking lot entrance has a sign for buses to use that lane but it does not say buses only. We noticed this lane was not backed up like the others and used it during our last visit.
We have arrived at the parking lot entrance booths before they opened but they opened up with enough time for us to park, enter the AK park, and walk to breakfast and still arrive before 8am.
I have read a number of posts about the parking lot not being open early enough but it has not been a problem for us. I would not want to make breakfast any later than planned though because we want to head for the rides as early as possible and don't want to be eating breakfast while the rest of the park guests are enjoying short lines and getting fastpasses.