TET,
One can carry a bit of Collective and lower "High" Rotor RPM.....if Rotor RPM is "Low"....well....there you are!
During maintenance test flights....the Autorotational RPM should be checked to ensure it is within the given Tolerance....with a personal opinion it should be at the high side of those limits if possible.
No one ever died because of "Too Much Nr"....but way too many have died because of the lack of NR.