Efficiency comparison between six step and FOC for a given motor
Field Oriented Control (also known as Vector control) of BLDC motors can improve low speed torque and reduce torque ripple at speed. Normally it uses sine wave drive. This works best when matched to motor with sine wave back-emf. However most motors used in multirotor drones are designed for 6 step trapezoid drive. 

The following traces show the different back-emfs of a 2 pole coreless ironless BLDC motor and a multipole iron cored motor. The second example might actually perform worse with sine wave drive.





So to get the best out of FOC drive it needs to 'tuned' to the motor. This might explain why the only two FOC ESCs I could find for sale (DJI 1240S/X and EMAX WindTalker) are both intended to be used only with a specific motor.

The vast majority of ESCs used in multirotors don't even use synchronous rectification, so their part-throttle efficiency is already ~5% worse than it could be. Also many of them are not using the latest most powerful processors and high speed drivers, so their switching losses are higher at high frequency (this is more important when using FOC drive, because to produce an accurate waveform requires high frequency PWM).  

The only one way to find out what real improvement FOC drive could make is to try it. For a fair comparison you would have to use the same ESC and only change the commutation technique.