I think the trend in car audio is to get away from capacitors. They work well when they can cycle fast enough to keep up with the bass spikes, but in my experience they don't make a big difference after the first couple of spikes.

What has made a big difference for me is upsizing my wires from the alternator to the battery, alternator to ground, and battery to ground (commonly known in car audio as "the big three"). There are calculators you can use to determine the minimum gauge wire for peak power draw. I used 0 gauge (it's overkill, but it looks cool and gives me capacity to expand). I would do this before upgrading anything else.