V_MBE speed is in reference to Maximum Brake Energy. The maximum speed an aircraft can initiate a Rejected Take Off from and remain within heat limitations of the braking system. The job of the brakes is to convert the kinetic energy into heat. They have a limited capacity to do this and, once that limit is reached the brakes fade. V_MBE is, therefore, maximum speed to reject the Take-off within the energy capacity of the brakes alone. If the take off is rejected above V_MBE regardless of runway length you will not stop.

On an upslope situation, the component of weight acts in the direction of drag (backwards along the longitudinal axis). This will result in a reduced acceleration force, helping the aircraft to stop in case of a rejected take-off. As a result, the brakes won't absorb so much energy which in turn will allow an increased maximum take-off mass.