librahall, I'm happy to hear you found this thread (and concept) interesting.
Interesting source for the SWR. I agree that it looks like PP is able to sustain higher withdrawals due to the added stability. However I think we should consider that gold has only traded freely since the 1970s, so there is limited pricing history for it. There is a much longer history of data for stocks and bonds, so SWR on things like 50/50 or 60/40 can probably be stated with higher certainty.
Since the historical SWR analysis with gold is only based on 48 years of gold prices, there may not be enough history to be very confident.