A capacitor would only help if you're experiencing drastic drops in voltage on dynamic ranges. If you're steady at 14.4V and then dropping to, let's say, 12.8V, then a cap may help you. But if you're dropping to 14V, that's simply a waste of cash.

Primary source of power when the engine is ON is the alternator. And unless you're a showcar, keeping the engine ON during SQ auditions is the preferred method. In that case, best to upgrade the alternator to a high-output alternator rather than getting a battery. There are a lot of aftermarket alternators out there. A general idea is that a stock Civic has a 70-80amp alternator, midsize sedans 110amp, if you're power-hungry, there's even 250amps out on the market.

Regarding the voltage, try to keep it around 14.4V. You might think that 14.5 or 14.7V is healthy but in fact it's not as normal systems aren't supposed to be charged with those. Lower voltage = less efficiency = more heat for the amplifiers. So try to keep your amps happy by having a good electrical layout

For a heads-up, if you're mainly into SQ, putting a capacitor is a waste. It's like buying ice when your drink is already cold. If you're into LnC or SPL, you might want to consider them for purchase.