You do realize the purpose of a cap, yea?
A capacitor is linear in your power chain. It's not like you're hooking a giant AA battery up to your amplifier that runs until it's drained and then recharges while you drive around.
It's function is only to provide extra power in your voltage chain when your amp needs it, so that in instances of bigger draw your amp pulls from the cap instead of your alternator which has every accessory drawing off of it. It provides clean power in instances when it's needed.
A farad is just a measure of electrical capacity. The more farads your cap is, the more voltage it stores. One amp drawing low power would benefit the same from a 1f cap as it would a 15f - it only pulls what it needs, when it needs it.
Having a cap in the chain doesn't (or shouldn't, if it's working and installed properly) effect amp performance at all when it's not needed. I'm not saying that it's sitting idle - stuff like your air conditioning, heater, etc -draws current and amps from your alternator, which without any sort of current control makes it's way to your amp, since your amp is part of the electrical system in your car as soon as you give it power and ground.