Don't know about the 206, but in the UH-1H (205) we commonly beeped the N2 back to 6400 from the usual 6600, and therefore reduced Nr as well, (all within flt manual limits though) and it seemed to save a bit of gas.
Don't think it gave us any more airspeed though, as I remember.
The explanation that the old and bolds gave was that at that speed, the rotor was running in a more efficient aerodynamic state, and that the usual setting (6600) was for a smoother ride. (Also a bit more tail rotor RPM too, obviously you'd want to be at the higher setting in the hover).