We had an 815am ADR at CP on our trip in December. We got to MK around 720am hoping to get in and have lots of time in front of the castle. Nope! We waited in line at the breakfast ADR line(s). You can get through security, but you have to wait at the entrance gates (far right hand side) until they let you into the park at 745am.
Be sure to check in right away when you get to CP. We did and we were seated right when the restaurant opened. We were like the 4th family seated!! It was great. We had no wait in the buffet line, were able to get our food and pretty much finish eatting before the characters came around.
If you want to finish before rope drop, make sure you take care of your bill early on in your meal, especially if you are on a dining plan. It took us at least 10-15 minutes to finish up the bill before we were able to leave, which made us late for rope drop. (We still hit Dumbo first with only a 5-10 minute wait!) If you get this taken care of early, it's very possible that you can make or beat rope drop!
Enjoy your trip!!