I would ask, is the test that Nigel performed is unique to his engine with the higher compression pistons? How about his induction, exhaust, propeller, and fuel delivery?
Ok, so you're suggesting the existing data might be anomalous, but apparently have no alternate to present.
The good news is that you happen to have an available airframe, engine, and prop which are totally different from Nigel's, equipped with P-mags and an EI Commander, which you know how to use.
There is data available. Here's what Michael Robinson found...with an entirely different airframe and engine, stock compression, and different RPM:
http://www.vansairforce.com/community/showpost.php?p=1054086&postcount=44
Short version:
100% power (0 MSL) 31.1"/2700 no power difference from 20 to 28 degrees
Cruise at best power (8,500 MSL) 23"/2300 best timing 27~28
Cruise at Peak EGT (8,500 MSL) 23"/2300 best timing 30
Cruise at 50 LOP (8,500 MSL) 23"/2300 best timing 33
Parallels Nigel's observations very well, i.e. advancing past 30 or so is entirely non-productive when ROP, in particular at high power settings. All you get is less HP and higher CHT...yet the stock P-mag advance was delivering 35 degrees at 23"
My own observations (which I'll be expanding in due course) are that when running ROP, my 8.9
angle valve motor may prefer even less advance. At 8500 and 65%, there is no detectable speed difference between 23 and 27 degrees, just a CHT increase, and that increase matches Nigel's observation of 2.5F per degree of advance.
Recall that many certified angle valve motors have a choice of 20 or 25 degrees, and Lycoming backed timing down to 20 for a lot of them, including the 390. Maybe it's because they have a dyno, eh?