Yes but that was the reboot problem. It will not make your servos slow down to the point that you can see the diff in speed. When the voltage is that low the rec will just quit and start its little reboot action. Actually I found out by putting my 6100 on a variable power supply it started to cut out at about 4.5 to 4.4 volts that is much less then any bec on a ESC is putting out and since he is running stock Esky size servos were not looking at a great current draw here 4 little servos and one gyro (servo) is really not going to hit 2-3 amps on a little King now if he was running say a digital servo and a 401 gyro I could see it if the esc could only put out 2a and he was doing some serious flying such as 3d where the everything was under a load all the time.
I run 3 hs-65 and a my gyro is in HH with the gain up right now so it is busy and my ESC is only rated at 2a. According to there specs I can only run 3-4 servos but in real life I am running 5 if you include my gyro as a servo and I have no problems and I doubt of many others do as far as current draw. A bec is actually more effecent at a lower output voltage then at a full batter voltage as it has to dissapate the the higher voltage pushed into it as heat then at a lower voltage which produces less heat.
I have tried a stepping ubec on my king and noticed no diff in servo speed or performance and I know of another king user (Zimatosa) that ran a Ubec (for about the same reason) and went back to to the built in bec after noticicing no diff in performance; but like I said if I was running 4 digital servos and a say 401 gyro it might be a diff ballgame
Myself I have a hard time watching servo go from one stop to the other and counting the mili seconds it takes and being able to say that it is slower now. To me a 1.8 second servo moves as fast as a .09 servo when I eyeball it.