I think you are right John, but its only a small amount. Since the battery on the firewall is about the same distance ahead of the CG as the baggage is behind the CG, a 10 lb heavier battery will allow about 10 lb more baggage without affecting CG.

I have the Aerovoltz lightweight battery and I nearly always go over max gross (1320 lbs) before I go out of CG range no matter how I configure my load. So I would make my battery choice based on other factors. The thing I really, really like about the Lithium battery is the way it holds charge. I can let the plane sit for a month or more in the winter and it will not lose even a 1/10 of a volt. I never hook up a trickle charger or "battery minder".