Torque values are also provided by JR at 6V...What makes you think you cant use 6V?
All the JR digital servos (mainly 8321 and 8511) that I have bought from the UK come with the following note (directly copied):
Please note that JR servos should be used with 4 cell (4.8 volt) NiCad batteries only. 5 cell NiCads MUST NOT be used.
The note is stuck to the outside of the box that the servo comes in. I am pretty sure that it is put there by Macgregor, the UK JR distributor, as the wording is identical to a page on their website. Why would they do this if 6V is OK?