Power is measured in Watts (P=V x A), not volts(V). Unless you have an ammeter (A) on your alternator, just knowing the voltage of the system is not going to tell you the load on the alternator.
The alternator in your car is PWM (pulse-width modulation) controlled, meaning the field strength varies based on demand determined by the powertrain control module. Using an under-drive pulley, for example, in this system could lead the PCM to command a stronger field strength more often, defeating the purpose of the reduced RPM.
Nice car!
