You said it yourself...about 4 hp for a 100 amp alternator. A 4 hp shift is effectively negligible gain. On the dyno, both my Mach and my '66 would show more than a 4 hp swing on back to back runs. You will get a lot more bang for your buck by putting effort into other parasitic drag devices, like going to an electric water pump over a mechanical pump. I've got one of those on the Mach, and they have show to increase hp (Dyno proven) on the 32V 4.6 between 11 and 17 hp, as well as having the benefit of better cooling at idle. Another choice would be to go to an aluminum flywheel (if you have a manual tranny). Less rotating mass=more hp freed up to the rear wheels. On my Mach, we added an Electric water pump, a C&L CAI, an intake spacer, a Borla cat back exhaust, an aluminum flywheel, and a custom tune. That combo gained over 30 hp and over 40 ft-lb torque. The intake spacer and aluminum flywheel added nothing to the top end #'s, but significantly increased midrange torque.