CW,
The human body can sustain 30G without permanant damage and 60 G instantaneous without permanant damage. Ejection seats are deisigned to these limits, although modern ejection seats have a peak acceleration of somewhat less than 30G. Therefor 20G seats are indeed s safety feature.
The 61kt rule actually comes about from a risk analysis which takes into account loads during a crash etc. It isn't just plucked. IIRC for every five knots above 61kts that you "crash" at the risk of serious injury/death doubles. (It might be ten knots, I can't quite remember way back to my course on certification and crashworthiness, although I am sure someone here can correct me if I am wrong!) Suffice to say this is one of the reasons why military aircraft have ejection seats. The military only accepts a slightly higher risk of death of aircrew in the event of engine failure than civilian certified aircraft, thus you will notice most single engined military aircraft (and all modern single engine military aircraft) which have a stall speed greater than 61Kt have bang seats. Transposing this into the civilan certification, 20G seats mitigates the higher stall speed of a PC12 (or other aircraft) and in effect the body in one of these seats will be subjected to the same forces as a "regular" seat at 61Kts.
Cheers
Mr B.