Many of us have found that what’s “trivial” is the small percentage loss in airspeed compared to the relatively large percentage decrease in fuel flow.
Exactly !
My quick-and-dirty numbers average out to a 5% loss of airspeed in return for a 40% decrease in fuel flow.
That seems like a no-brainer. Particularly when LOP eliminates the need for a fuel stop.
“My quick-and-dirty numbers average out to a 5% loss of airspeed in return for a 40% decrease in fuel flow.”
From full-rich, sure.
The only other way you will get a 40% less fuel burn for a 5% loss of IAS is if you drop from somewhere around 100% power to 75% power 
My TB20 does 165kt at 23 GPH (max power) and 140kt at 11.5GPH (65%). But that’s a silly example because nobody is going to be flying at the former setting, unless the airport is closing soon.