A capacitor is only good so far as the audio system isn't trying to pull too much from the electrical system. A cap doesn't provide more power, it's designed to 'stiffen' the voltage to the amp, nothing else. If the current isn't there, a cap won't help. The alternator must have at least 20% more amperage power than the entire vehicle and sound system combined for a cap to be of benefit which is ironically the same requirements for an amp to be efficient. A cap does it's best to keep the voltage at a certain level, but if the voltage changes so will the cap voltage. The cap just makes the change smoother. The diming light syndrome isn't a voltage issue but rather a current (amperage) drop. The voltage simply follows what the current does. A cap does not help this situation at all. A cap doesn't provide more power nor does it try to correct it.
go extra battery route, youd be getting the most for your money, cuz oodds are youd end up getting a battery anyway eventually...