Matt,
A cam driven pump or for that matter any lift pump (belt driven?) that changes flow with engine speed would need to be greatly oversized so that the full VP44 demand would be satisfied at idle flow rate. This would mean a lot of extra fuel would be pumped and any rpm over idle would have a lot of fuel being to be returned to the tank. Think of what happens when your engine is at 3000 rpm (stop light racing) then you need to slow quickly, push the clutch in, instant idle speed but the flow demand at the VP44 (CP3 also) can actually surge higher momentarily. Not a good situation with a engine driven pump. With an electric pump there is a constant flow regardless of engine rpm.
The 12V pumps are pressue and flow regulated on the back side of the P7100 with a return valve that is set ~30psi. If I remember correctly idle psi with a new pump is ~20psi. The P7100 is a lower demand injection pump then the VP44. The VP44 requires substantial flow backto the tank for cooling and lubrication. The P7100 is cooled and lubricated by engine oil pressure. All the savvy high horsepower 12V engines that I've seen have dumped their stock piston lift pumps for electric or hydralic style lift pumps.
If you did choose to use the mechanical/plunger pump it would be necessary to put an accumulator in the fuel system to keep the pulsations from hammering the VP44 and a large regulator/return line back to the tank. The 12V lift pump exceeds the reliability of the 24V Carter but it's not unusual to replace the 12V pump between 80,000 and 125,000 miles. Better but not bullet proof. HTH's, Larry
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