There is no correct answer to this. Yes, many times you can get in around 7:45. And sometimes they let you in about 15 minutes prior to your ADR. But, there are some days that they don't get those guests in even by their ADR time. The reason for the latter is that sometimes, they are still training and/or rehearsing on Main Street and won't let anyone through the train station because the "onstage" area is not ready for guests. Earlier this month, I waited in a very long line of early entrants until 8:10 when they finally were ready to let people in. The 8:00 ADR people and the 8:00 tour people were, of course, "late" to their experiences and therefore had no time for pictures.
I would recommend you get there by 7:45 and they will let you in when they are ready.
