[mwebb]

..."I'll happily trade high BSFC conditions for low load, low rpm conditions.
35-45% @ 2000 rpm gives me best real FE."...

since you have a diesel engine ....
under that condition your system is most likely operating with around 50% EGR ...
and
your system probably does not have a throttle plate
( if it does have a throttle plate it mostly uses it to create a better difference in pressure between intake and exhaust to enhance EGR flow )
so it always operates at a much higher VE volumetric efficiency than systems that have gasoline engines and throttle plates .

apples and bananas
=================================
Scan Gauge does show real time Fuel economy , just drive while monitoring real time fuel economy to maximize your own results
different
but same end results , you get better fuel economy , use what you have .
if you do not have scan gauge , do not buy it , as ultra gauge and OBDguino or MPGuino can show real time FE
for MUCH less money .
same results for less expenditure