Here is why that high-current alternator may not be all you think it is. In order to keep the size the same there are fewer windings of the fatter wire to handle more current. That makes the low-rpm performance actually worse than the stock alternator. A road car spend most of its time at idle and low rpms so that is where you need the alternator to perform well. At idle stopped in traffic your fans are going full tilt and your alternator is barely turning. This graph explains it better. Below 2000 rpm the high current alternator performs worse than the lower current alternator.