You'll see that if you get to high RPMS then engine is less efficient. The lost energy can/will be just be wasted as heat. (And charging is always have some loss, so charging via MM is only going to help if you can do it at/near the ideal RPM. You can quickly loose
If you don't know the RPMS, you can definately be less efficient. If you push it to 4500RPM you'll be at about 280 g/kw which is almost 20% less efficient than the 2100RPM.
The first law of thermo is not conservation of energy.. its about heat and work being transfer of energy and in a closed system they interchange .. if its not useful work it will become heat. Second law is increase in entropy (except at thermal equilibrium )