If you know how Power is stored and discharged, you also know that a capacitor only catches up the peaks in the current-demand (but most amps already have capacitors inside to do this), but raises the constant current-demand.

The only difference is: without a capacitor, you're kicking your alternator and battery untill it finally dies and with a capacitor, you're letting it die a slow and painfull death.

I'm pretty sure your alternator will die one day (probably taking the battery with it) and I'd recommend you to get an upgraded one + a better battery (an Optima Yellow Top isn't exactly the finest quality, it's just well known...) and then try out the install without the 15F capacitor in between.
I'm sure the lights won't dim anymore and as long as the battery is fine or you replace it in time, and you don't add Power to the system, the upgraded alternator will last untill the carbon-brushes are worn out.