I did some research and the only reason they say not to use it with 6v is that it's designed to be used in helis with gyro's and the higher voltage will damage the gyro. 6v for the servo is fine, it's the gyro that can't handle it. I also can't find any torque specs at 6v but it's got to be over 100 if it's 92 @ 4.8v
**taken from the site you linked me to above**
1) Always use a 4.8 volt NiCd w/gyro. Dry cells are rated at 1.5 volts each making the overall voltage 6.0 volts which will damage the **gyro**. The servo operates fine on 6.0 volts.