The fuse has nothing to do with how hard your alternator runs, that's the job of the voltage regulator in older cars, and the pcm in newer cars. Basically the alt does what the regulator, or pcm,tells it to do. If the pcm see's low voltage, it will demand more out of the alt, and when it see's a resting 14.4volts, it will make it run just enough to keep a steady voltage. All fuses are for, is a safe guard against shorts, or voltage spikes, so nothing catches fire, they don't restrict, or control anything except braking the chain of juice, if to much is traveling through them.