Battery 'upgrades' in car audio are worthless whether it be multiple batteries (more strain on the alt to recharge) or more expensive (nothing beats a lead-acid for current output-yes even Optima red/yellow/blue top). The only reason to do anything with the battery system in your car is if you're running your system with the car off often.
14.4VDC comes above idle, from the voltage regulator attached to or as a part of the alternator. at idle the car is 12.6VDC typically.
you need to upgrade the charging system if you want 14.4 volts, but if your amps have regulated power supplies, it won't make any difference, and even with unregulated supplies, you're only gaining about 10% in power going from 12.6 to 14.4VDC, which really isn't an audible change anyway.
At best it might make a difference on an SPL meter of a tenth of a dB or so.