For whatever my view may be worth, I see no problem using an old P chart (generally with which I am comfortable) provided that I also check that I am not unconservative with whatever prescribed data is the flavour of the month.
That is to say, I see no problem using the old P charts to add a sensible level of conservatism to the light aircraft POH data.
CASA and the judge will only be interested if you were to operate non-conservatively with respect to whatever the requirement may be .. it is up to you as to how much, and by whatever technique you might use, conservative fat you add for mum and the kids.
I suggest that the regulatory harmonisation process was not about CASA's liability at all but a consequence of studies which concluded that the benefits to be had by removing local differences were a valuable goal.
Whether one might agree or not is the province of one's own assessment.