AFAIK, racers use smaller alternators to provide just enough power to keep the battery charged and provide for lighting/ignition. And in your case, your engine is spinning more than usual road car. I don't know much about underdrive pulleys but my brain is saying wiring problem or simply bad alternators. Genuine Mitsu alts are known to be fairly durable, internally regulated. How are the alts dying - mechanical failure from high RPM operation or electrical failure like the regulator getting fried?