You can definitely finish the character breakfasts before rope drop if you are seated at the earliest ADR (usually 805am). (Ohana at the Poly had early reservations and is a short monorail ride to MK). We were seated at CP at 805am in December, and finished at 905 as well. However, it took a really long time to take care of the check. My advice is to ask for your check early and get it taken care of before you are finished with your meal, especially if you are on the dining plan.
Usually the characters come fairly close together, so it generally doesn't take too long to get through the characters. However, you could get some late comers as well.
If you are doing a buffet but are seated quickly after the place opens you will more than likely not have to wait long in the buffet lines. We were seated at CP at 805am and were told by our waiter to quickly get in line before they got too long. I don't think we waited at all. We were like the 5th family seated. It was great. By the time we thought about seconds, the lines were crazy, so we just grabbed a made to order omelet because the line was not long for it. (That was a delicious omelet.)
Have a great trip!