I thought a capacitor was just to help get a motor started. It doesn't increase the speed of the motor after it does so. I've never heard of people with electric/hydro units running capacitors. This is something new to me.
According to Crutchfield, "In car audio, large outboard capacitors, sometimes called stiffening caps, are used to prevent lights from dimming when loud bass notes play. They accomplish this by supplying the amplifier with a quick jolt of power."