I'm new to car audio stuff, but I've been in electronics way too long...
In an electrical sense, caps simply provide bursts of energy on demand for very short duration (a second is too long) ... after which there is a "recharge phase" in milliseconds... depending on the cap before it can produce another burst of energy... (i'm talking about capacitors in electronic circuits but I'm assuming that the caps used in cars work the same way)
Now if there is dimming only during sub hits... then caps could definitely help... however, in order to provide "continuous" bursts, it has to be recharged constantly... therefore the alternator which produces the power, must be up to providing said power on a reliable basis (meaning able to produce extra energy on top of the regular demand of the entire system priot to the sub hit)
If the alternator is weak to begin with, then you have a problem... your cap will work only with the first sub hit... after which the recharge time gradually increases to a point where the caps will only work intermittently (or worse fail) ... secondary to insufficient power being given to the caps itself and to the entire system.
However, the cap theoretically helps to protect the alternator and therefore stabilize the entire electrical system by providing the burst of electricity needed by the sub amps... rather than ask the alternator to produce more electricity evertime there is a surge in demand, the caps will attempt to provide the power and the alternator can just go on producing a constant rate of power as if there was no increase in electrical load.
that said.. it seems that the saying... the system is only as good as it's weakest component holds true... you have to have a strong alternator and a good capacitor to maximize the benefits from the setup. One does not replace the other but theoretically, if you have an oversized alternator (if such a thing exists) then you don't really need a cap.
I may be wrong in all this as I said, I'm new to car audio specifically and I'm not sure all the theories are directly applicable in small circuit boards as in cars....//content.invisioncic.com/y282845/emoticons/banghead.gif.8606515f668c74f6de0281deb475b6fd.gif