I know there are varied opinions on this, but having done this for a living for a few years(in the past), I pose this question: If caps were just "expensive led number things", why in the world would your major competitors, sponsored or not, be running them? The last time I checked the rule books, they have installation guidelines and so forth, going as far to say that 6 caps in series qualifies as a second battery.
There is not a battery, alternator, or combination of the two, that will charge as fast as a stereo demands. Even if you don't have light dimming, you have voltage drop, and that is just about the fastest way I know of to burn up equipment, or at the very least hurt it's efficiency and productivity. Not only will a cap make it easier on the existing battery and alternator, it will clean up the sound of a system. It is usually more apparent it the mids and highs sound than in the bass, but I have heard several systems benefit from a good cap while just running sub amps. Plus those "expensive led number things" give you a way to keep you eyes on your voltage, instead of our factory analog no number gauges that I have personally found inconsistencies and delayed readings and so forth. I mean, I know I'm a little more anal about this than most, and equipment design is better nowadays, but to me it's like insurance.
Not trying to start a debate here, but the facts are the facts. They are not "snake oil", or anything of the like, they have a certain purpose and function.
I wouldn't run over 500w without one, but like I said I'm a bit anal. Most seriously consider 1000w as a threshold of cost vs benefit.