Been ruminating on this one. It would be theoretically possible to increase fps with higher battery power and vice versa. When dealing with such high speeds (almost everyone's gun cycles 10-20 times per second! When dealing with such speeds there could be significant "throw" of the piston that actually gains a couple % more compression than when the sector gear is slow moving. 3% more compression would yield the 10 fps change that Jagdalex has seen.
I now own a high end chronograph, the CED Millenium2. I intend to test this; indoors so there are no temperature or wind or lighting variables.
I am glad you posted Jagdalex, because your data backs up the theory that I was going to post about this morning anyway. M102404 brings up a good point about lighting, barrel angle, etc in his post so I will do my best to eliminate these variables. The indoor location will remove most. If I use a "bench rest" in front of the chrono and just swap batteries from 7.2v to 8.4v to 9.6v I should eliminate the rest. Then I will post the results.