From simulated engine failures (with fixed fuel flow on remaining engine) my experience is that one very important effect of drooping NR is to use part of the kinetic energy in the rotor system during the initial part of the flyaway. Even for a machine that had identical performance at nominal and reduced NR, it would be very beneficial to reduce NR during the flyaway.
As soon as conditions allow keeping torque within limits, I would think that higher NR (more power) is more important than a small increase in main rotor efficiency.