Such a setup can be more efficient - but only if everything is tuned perfectly. ICE engines are generally most efficient at low RPM and high throttle settings (last time I saw the graphs it was 1950RPM at 87% throttle peaked at 43% efficient, but that was one particular engine: yours will be different) - so you the entire system tuned around this, mistakes mean you either are running lower throttle or spinning extra mass not needed. Very few companies have the ability to design an engine around their exact needs - only the large automakers can really pull it off and they typically trade engines with other auto makers so they can offer more engine options in their cars than they have designed.