Kyle & Berto article about transmission losses etc. (plus other interesting articles)
http://www.ihpva.org/pubs/HP52.pdf
Rohloff's answer to Kyle & Berto article
http://www.ihpva.org/HParchive/PDF/hp55/hp55p11-15.pdf
Here is Rohloff's own article about transmission efficiency
http://www.rohloff.de/en/technical/speedhub/efficiency/