The difference in maximum speed with only one AFCS operational is due to pilot intervention time if an AFCS runaway happens. With two systems working, if one fails (let's say a roll actuator suddenly goes to full right roll), the other system can take out part of the effect - the pilot has to be 'warned' of the failure - which, like engine failures, will take some finite time to be recognized and then for appropriate action to be taken. It''s much worse if you only have one system working, and it decides to runaway - the pilot recognition and reaction time will be the same, but there is no other AFCS to counter the effect of the runaway - at higher speeds, the failure could be more than entertaining - hence the lower maximum speed.