I borught that very question to Horizon Hobby last year. I was considering running a six cell pack in my airplane. So I asked if I could run an 8411 at 7.2 volts. His response was NO!! Don't do it. Then to add some insult to the guy I said, wait a second, when you charge a 5 cell pack it charges sometimes to well over 7.5 volts, so why can't I run a 7.2 volt pack.

He then put me on hold to consult with more knowlegable people.

The answer came back "no more than a five cell pack".

So you are good on a 5 cell pack charged to whatever voltage it will hold at least on JR's servos.

Sean