Tests on the APM2 I've got show at least that:-
1) The board complete with xbee starts at 4.41V and at 5.63V.
2) At these voltages it will run for at least an hour.
3) The GPS locks and stays that way.
4) That after the hour there is no more drift from the sensors than at 5V exactly.
I'm going to power this board at 5.1V and the servo's at 5.8V!
I'd hoped to have an offical answer, what the board tolerates is not it's specifcation.
Really a board of this cost should have at least an electrical spec that states the limits for the electrical connections, it's not much to ask. Even cheap chips and boards mostly manage this.