I don't mean to be critical but, I simply don't understand why you'd ever mount your battery forward of the firewall. That seems like a very unfavorable thermal environment for any battery. Unless CG is an overriding consideration, why not put the battery in the cabin somewhere? Concerns about gassing, acid leakage, etc. seem to be largely mitigated by a properly maintained AGM / Lithium battery so I'd put that sucker in the reduced vibration, cooler environment where, hopefully, it will be more happy.
The other thing that AGM battery owners should consider is the charging voltage. The charging source for an AGM battery should be set to 14.5-14.7 volts. That's much lower than most folks probably have their regulators set for their standard lead acid batteries. Most Lithium batteries have their own charging circuit and will live happily with a wider range of input voltages. AGM batteries, with no integral circuitry, rely on the charging system in the airplane to supply the proper charging voltage. If you have an AGM battery like the PC680, you should check your regulator to ensure it is set for 14.5-14.7 volts.
As an aside, I also think that most of our aircraft electrical systems are ill suited for the proper maintenance of any AGM battery. In my motorhome, my charger uses a very different charging and maintenance process to maintain my AGM batteries. In my airplane, there is none of that level of sophistication. We just feed it voltage and current for the duration of the flight and hope for the best. That's not the best approach to battery charging / maintenance. AGM batteries like a fairly precise sequence of bulk, absorption, and then float charging. The lithium batteries that I maintain for my radio control aircraft also require a fairly sophisticated charger. Where is that level of battery maintenance sophistication in our airplanes? Maybe an opportunity for some aspiring entrepreneur!