We had 815am ressies in December. We got to MK around 7:25, went through security and waited in line at the character breakfast turnstiles (they were all the way to the right hand side) until they let us through at about 7:45am (some people say 7:50am). We stopped and bought autograph books at the small store just inside that was actually open (yeah or our girls would have had nothing for Pooh and friends to sign). It was kind of crowded in there because people were also buying ponchos (it was raining lightly) so it took a little while.
We walked down main street towards the Castle and CP (hardly anyone around - very cool). We got there around 8am or a little before and checked-in. We were seated right at 8:05 - like the 5th family seated!! So, definitely check-in as soon as you get there because if you get in early, you get to the buffet lines with no wait, get to enjoy most if not all of your food before the characters start coming around. We took pictures of the castle while we waited after we checked in.
You should be able to make rope drop. We didn't, but we learned a valuable lesson. If you are on the dining plan, ask for your check very early on in your meal. It can take quite a while (over 15 minutes in our case) to take care of getting the bill with pricing on it (despite telling your waiter your on the dining plan), getting the bill for the dining plan, giving a CC to pay for the tip, and getting everything back to sign. We would have easily made rope drop because were done (characters and food) by 8:45am. So if you get paid up early, dining plan or not, you can get up and leave when you're done and not have to wait to take care of your bill then.
Despite the wait, we walked out a little after 9am, took pictures with a photopass photographer outside the restaurant and another one outside the castle, headed for Dumbo and only had a less than 10 minute wait for our girls favorite ride.
Enjoy it! The make-your-own omelets are delicious. The other food is really good too.