One facet of ancillary systems that popped into my head, though not related to engine power diversion, was the use of externally powered electrical generators. This particular to German construction practise. Pilots of the Me-163 and Ju-87 mentioned that the drag of the devices propeller, as well as the centripedal forces, tended to steer the aircraft at high speeds.
I recall reading some papers done on the topic of engine power losses by NACA. I believe the author was Weick, though not certain. It's been a while. Oil viscosity figured in the investigation. Both the U.S. and a U.K. site have the papers internet available. Regards