[HypermilerAX]

If he's at 10% higher load with 12% less revs, it means that the conditions were not the same.
Power = torque*rpm so if you lower rpm by 12% you will necessarily increase torque by 12% to have the same power. With a higher gearing, you only gain with the second thing you mentioned: efficiency.

My 2 re-gearing experiences showed that MPG gain is about 55% of the gearing change (based on averagely geared cars, not too high nor too low originally). This is valid at cruise speed in final gear. 10% would translate to about 5.5% gain. But when driving always in 5th gear. Try to estimate how much you use the last gear in regards to the total distance.