Sims are fine but is your Sim actually duplicating the reaction of a properly Rigged aircraft?
Also...what are the Transient Limits on the Aircraft you fly?
Most are very tolerant of transient over speeds....as there is a time limit associated with the Over Speed Limit.
Dig into your Aircraft Maintenance Manual for computing the Power Off Main Rotor RPM's for a given Aircraft Gross Weight, Pressure Altitude and OAT....and see what procedures are involved. Ensuring the Aircraft is tested in as close to the conditions as possible to how it will be flown operationally is important as well.
I will bet you that in reality it is very hard to harm an Aircraft if it is properly rigged per the Approved Maintenance Manual. My experience is that particular maintenance effort is not given the attention it should and close enough is good enough for far too many Operators.
The operative words here are "Properly Rigged".
My experience on over a dozen different Types tells me I am right. I have done all sorts of CofA Air Tests, Maintenance Test Flights, Production Test Flights and have never had a problem so long as the aircraft was properly rigged and flown.
All it takes to avoid any over speed is proper application of Collective....if the Main Rotor RPM starts to go high....simply input a bit of UP Lever....and that problem is solved.
The Danger is getting the rigging wrong and not having enough Autorotative RPM as there is not much One can do to remedy that situation besides turning or flaring but those gains are only temporary and result in either an increased ROD or loss of Airspeed....both of which are not desirable in an EOL situation.
I always tried to set the Autorotative RPM at the High side limit for that reason.