I am an engineer and I would say that claims of 95-97% are not accurate. The most efficient transfer of mechanical energy is not with intermeshing gears. Any claims much above 92% are very suspect.

>>Schlumpf claim 95% - 97% efficiency for the SD (although Greenspeed noted power losses with the Speed Drive in top).<<

A pair of IDEAL gears [absolute perfectly smooth, polished faces etc] with absolute minimum losses [no losses to lubricant heating and churning or extra moving parts etc] would not exceed 97% efficiency. Schulmpfs are good bu not perfect.
So each gear added would indeed drop the efficiency by a minimum of 5%. having the chain between the front and rear also causes around 10-15% losses [clean versus dirty] so for a drive train with front chainwheel, chain and rear direct drive the max effiecincy might be about 84-85%.

Adding the Schlumpf would probably drop a minimum of at least 5-7%. So the losses in the drivetrain could be near 25%, 5-7% attributable to the Schlumpf at least.

Slim