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Adding boost to an engine will never increase the efficiency of the vehicle, even if it is from a 100% free source, which is never possible anyway. The only time boost helps is if you use a smaller than normal engine boosted to "normal" power levels.
Put it this way: an engine with a supercharger and an identical engine without a supercharger at the same load (say steady state cruising) will have the same manifold pressure (ie vacuum) at said load, but the pumping losses will be greater with the supercharged engine because it will take more throttle restriction to maintain the same manifold pressure.
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