Odyssey and CS prop are not ALWAYS an issue.
I'm doing the same thing -- an Odyssey battery on the firewall. And I'm using a CS prop. My plan is to modify the stock Van's battery tray to take the smaller battery. The firewall location seems a lot easier to access than if the battery were in the bottom of the forward baggage compartment, for example. Not to mention the hassle-factor of running the heavy-gauge wire back to the aft baggage compartment.
As Rob mentioned above, the C.G. with the forward battery, an angle-valve IO-360, and the Hartzell CS prop can be an issue, but I plan to use a lighter composite prop and a lighter parallel-valve engine, plus I'll have a smoke-oil reservoir and pump in the aft baggage compartment, all of which will negate the effect of that tiny battery on the firewall. My calculated CG is almost dead-center.
If you put your ELT, strobe power supply and a few other heavier goodies in the aft baggage compartment (or, even better, aft of it), you can actually make the numbers work out pretty well.
YMMV.