Absolute best use for a capacitor in an high performance audio system would be to get the expensive ones with 500 farad capacity and run no battery, or run the battery to start the car and do the audio all off the caps.. A really high end cap bank like the linked one, especially if you run 2 or 3 will outdo any battery you put in front of it, even the big group 31's by the big name companies (xs, NS, doesn't matter). T-fade did a test comparing them head to head to quote him "batteries don't want none" it was batteries dropping into the 11's and putting out over 1000 watts less out of the same amplifier as the caps. SPL guys figured this out a little while ago, at least the smarter ones did.
That being said, they still work if you wire them inline to your battery, but they work better when they don't have to deal with the batteries slower discharge rate. Nice thing about caps if they can send electricity out as fast as it can flow essentially so they are working all the time. Battery banks dont' really do ANYTHING until you get down to the resting voltage of the battery, which even on a good agm is mid 12's. Until then your alt is doing all the work. With caps, they'll charge at your alts level if wired directly in and hold that voltage as long as they can. If you get a few banks to give yourself some reserve, you have a killer setup that batteries can't match. Much longer life, much better performance and much better weight.
