Automotive alternators are inefficient because they're using a 50 year old passive overload protection design. You can put unlimited load on a generator with the field current at maximum and as long as the Voltage does not fall below 12 Volts it won't be damaged by overheating, at least not right away. There is a huge amount of stray inductance in the windings. At idle, it may be putting out 16 Volts AC but internally (if measured with no load) it would be putting out 28 Volts. At high RPM with 100 Amp load at 16 AC Volts produced, internally the alternator may be putting out over 100 Volts, if you were to measure it with no load. As the RPM increases, the stray inductance becomes more effective at reducing the output current.
If you can get 24 Volts out of an alternator with the engine at idle, you can easily get 150 Volts out of it with the engine at high speed. Obviously if it's producing 150 Volts but you're only getting 14 Volts out of it due to inductive reactance then there is a huge amount of inefficiency going on.
The stator is very lossy too. If you apply full field Voltage to an alternator you can even feel the resistance if you try to turn it with your hand without any load. That's all from the magnetic field being lost to the stater. Electrical current is being inducted in to the stator and it is generating heat. A huge amount of air flow created by the alternator turning at thousands of RPM keeps this heat under control. Some people install an alternator disable switch that activates at full throttle to not waste that extra horsepower.
Then the rectifier will drop about 2 Volts at higher loads. 16 Volts going down to 14 Volts is 12.5% loss in addition to everything else.
If you can get a whole lot more power out of an alternator that is turning at high RPMs by letting it put out a higher Voltage. Doing this would also make it much more efficient. The rectifier diodes are TVS diodes though so they'll keep the output under 40 Volts and they'll destroy themselves in the process if need be. Without TVS diodes, if the alternator was operating at high RPM and full load, and the load was suddenly disconnected, the output could spike to over 100 Volts before the regulator readjusts. So you'll have to replace the rectifier with a regular 3 phase rectifier and provide a way of handling surges. Then manually control the field current. Letting the output go from 14 Volts to 28 Volts with the alternator internally producing 150 Volts would nearly double the output power and efficiency.
Instead of passive overload protection they could make a more efficient stator and just have a current sense on the output to make the regulator back off to protect it from overloads.