I agree with you completely. All of my NA planes were most efficient around peak to -20 LOP. My turbocharged planes were most efficient around 50-70 DLOP depending on power setting. At 65% or less you are out of the red fin and can run as lean as you want, but going excessively lean is counterproductive.
With that said, I have received multiple replies where people are getting at least 165 knots and most are getting 170+ knots LOP around 8.5 GPH. The plane in question is around 147 knots. Now, this plane is likely a little heavy with deice, big extra alternator, 2 batteries, etc, but flight test was done with 1/2 fuel and just the pilot.
Even at 11.9 GPH it would only eke out 157 knots. Has to be a drag issue.