Well I am plan on going from POR to CRT breakfast ( 8am ADR Xmas EVE) i was worried about the same thing -how long is this going to take us??
I went on wdwtoday and the guys said that the bus would be fine to take and the taxi would take me to the TTC. They said double check the bus time with the front desk the day before.
All that said - Check with the front desk and ask them what your best bet would be as to when the buses start going the day you eat there with it being that early.