it is non-intuitive that a higher voltage to the servo could be worse. i think the problem is that with increased supply voltage, the servo unloaded rate goes up, but the current required for a given load also goes up. so if the circuit is current limited, a higher voltage may perform worse.
have already tried it. all my ko stuff is made for 2 cell. once powered from the internal bec, the sevo runs much slower. not a big deal, I have already got my solution and it does what I want. fast servo response and under 7volts to the speedo