Falcon, relying on any of the FMS predictions for assessment of fuel savings is a mugs game. The tool is very primitive to start with, uses different parameters and algorithms than a flight planning systems and the fuel systems also have measurement errors. The net result is poor data fidelity for inflight evaluation.
Using total flight data tracking smooths out the results and does demonstrate the savings from low CI operations.
We must also remember that typical holding speeds are also not shown as the pure max endurance speed to address the speed stability issue you raise, so comparisons of a FMS generated hold speed does not necessarily accurately reflect what the true Max Endurance speed.
I have spent 10+ years working in the field of fuel efficiency and CI. I work it from two angles, as a current line pilot that still flies and has flown multiple types from Boeing and Airbus, and from the engineering & fuel efficiency side. The number do not lie. Low CI operations are just different to what we are used to, the world has changed and we have to adapt to it.