Yeah but think of it this way. With a cap, you are not adding any capacity to your electrical system. With a battery you are actually adding more capacity to supply amperage.
I've seen caps be installed with no noticeable effect, or at most it stays a tenth or two higher during quick bass notes. Even that slight of an observation is subject to tons of other variables so who knows if that's actually an accurate. However I've seen batteries and even the big three make exponential differences in the electrical system. I did the big three in one car with only 4 AWG wire where it made the car stay an entire volt higher at all times. A volt is an amazing increase for 4 or 5 foot of wire. Caps may or may not be beneficial but honestly its just one more product for the car audio industry to sell to people IMHO.
What I heard is that capacitors are actually better for SQ systems because they act as another way to filter the ripple from the charging system. In amps caps are used for power storage... which in a good capacitor I don't think anyone denies because the abilities of caps allows us to have crossovers, amplifiers and tons of other things. Its just that they also need to bring that power from elsewhere and that they discharge so quick that their benefits can be argued