One thought: in making those measurements at peak EGT and 25F LOP, IO540, were you using the richest cylinder or the leanest? If you have GAMIjectors (sorry, I can't remember if you have), the spread between the two may only be 0.5 USG/Hr. If you don't, spreads are commonly in the 1.2 to 1.8 USG/Hr, and the 'average cylinder' is actually running quite LOP when set up for peak EGT of the richest.
I've got GAMIs, and yes there may be something in this, but equally I found that going much deeper LOP (say 100F LOP) there is no measurable difference, and the power loss is pretty significant.
I believe the theory more or less supports this, since power can come only from burning fuel, and once you are stochiometric (peak EGT) or just past, the only additional gains can come only from reducing the pumping losses, and if you are already WOT and at low RPM then there is nothing left to play with.
What I didn't make very clear in my post is that all this is with WOT (wide open throttle) which is not feasible at low levels, say 3000ft, because the engine power is too high. I have tried flying with WOT at say 3000ft but leaning deeply LOP, to achieve say 65% power, but the engine sounds pretty unhappy, so I don't bother. At low levels I fly 23", 2400rpm, 11GPH which is marginally LOP, and everything works nicely. Except that I am getting a good 10% less MPG than I would be getting at FL100, WOT, 2200rpm, 9GPH (also marginally LOP).
Of course, if you plan a maximum endurance flight, or a maximum range flight, the numbers in the POH should be used instead
The problem is that the POH was written in 1780, by Isaac Newton, before modern engine instrumentation revealed what can be done if the engine is set up accurately. So e.g. my TB20 POH says "best economy" at a certain %power and TAS is at 12GPH when in fact it is at c. 11.1GPH - a difference of about 9% and that ups the range from the POH value of c. 1150nm (from memory) to 1300nm which is what I actually get computed by the Shadin/GPS system on a real flight.
I really do not think that in all the years that these engines have been around, say 40 years for the old Lycos, nobody really paid much attention to these details. The instrumentation was not around. Only GPS made really accurate TAS measurement really easy, and more to the point only a flowmeter-GPS coupling delivers a constantly recomputed range / fuel at destination figure, without which you would not be confidently doing long trips, and if not doing long trips then you wouldn't bother optimising things in the first place and you would be flying just like most people do: stop when you need a leak
I suspect some of the old pioneers (Lindberg?) did suss this out, and used it as far as they could. Then, certified GA with its certified flight manuals covered in certified rubber stamps, took a step back in time, discarding this knowledge.