Forget about Caps. Spend you money where it will make a difference.
To get louder you may consider the following:
1) If you install more speakers, mount them as high as possible in the interior
2) Power those speakers with amplifers that put out close to rated RMS power handling of the speakers
3) Consider installing a couple of 10" or 12" subs powered by a dedicated Class D Subwoofer amp.
4) I'd increase your battery count by 1. One for engine starting and 2 dedicated to stereo