It is (theoretically) possible that the helicopter reaches such a high rate of D, that it becomes very difficult to flare it to a safe touch down speed-especially in small openings....as we all know, the engine(s) are likely to quit not at the final of a long runway, but more or less when flying over areas where we do not want to land-not to say autorotate into...
Overspeeding shouldn´t be an option-and that you never observed a real "overspeeding" indicates that you never experienced a "engine off"-autorotation in a fully loaded helicopter (as we did in Maintenance Test Pilot Course)......
The R-rpm IS going to overshoot.....if you do not recover.
As the FM already takes into account a higher R-rpm for autorotation, one shouldn´t play around with his luck....as he will need it at the final touchdown phase of the AR....