as far as i can see nobody has mentioned that the lower limit is 80% at which the governor will grab it, also I am not sure at what point it fails to work on the upper limit, but i believe it to be 110-115%,
I did hear a story of someone who overspeed one for about 2 minutes, but they were above the governor range and it didnt respond and unforunately they didnt have the presence of mind to roll the throttle at all and presumably have done some damage. (imagine what damage they could do starting a jetranger up!!!!)
also there was an accident recently where the tail fell off an R22 as a direct result of an overspeed, because as I understand it the tail rotor drive shaft starts "whipping" at about 130-135% because the harmonics and resonation have been designed out of the normal operating range and this whipping causes a lot of collateral damage, so that would efectively be your upper upper limit if you had a real engine failure and mentally you completely lost the plot during the subsequent auto and engine off landing.
a semi related issue, I have noticed that there appears to be a lot of resonation in the R44 at 75% rpm when running up, but am not sure where this is coming from, does anyone know?
elpirata