Modern fuel injectors DO increase fuel combustion efficiency. BUT…
Carbon-fuel (including diesel) combustion efficiency depends on the fuel turning to vapor and homogeneously mixing with air. That’s why the injectors make tiny fuel droplets and disperse the droplets throughout the combustion chamber.
Generally higher fuel injector pressures and tiny orifices promote smaller fuel droplets that increase the diesel fuel vaporization rate, promoting more efficient combustion but…
Atomized fuel is still liquid fuel and although it takes less time to vaporize, it still takes time; and it is not homogeneously pre-mixed with the air in the cylinder. Combustion happens at the surfaces of the droplets…
NO fuel injector (old or new), injecting liquid fuel, can beat the performance of fuel that is ALREADY vapor and homogeneously mixed with air when the injection (combustion) process starts.