Ah, macspud, that is the million-dollar question!
A careful scouring of the Fallbrook website (makers of the NuVinci hub) fails to reveal any hard data on the subject.
A Fallbrook-issued PDF ( http://www.fallbrooktech.com/NuVinci_Experience.pdf
) refers not to "mechanical efficiency" but the results of their "Fallbrook Experiment" showing increased ..."overall 'ride' efficiency" based on such factors as always being in the right gear and not having to suffer ride-delaying events like having the chain fall off (!) and the "...associated noise and clatter" of derailleur systems (!!).
The hubstripping blog ( http://hubstripping.wordpress.com/atc-nuvinci/
) has an interesting section and videos on the intricacies of NuVinci N360 setup and the comments section mentions difficulty obtaining information on efficiency from Fallbrook. It seems the company continues to refer to positive "rider experiences" and "the total smooth riding experience" rather than hard data wrt efficiency. Rider reviews only allude to the apparent "lack of slippage", but these are impressions, not empirical data and in no way reflect any objective measure of overall efficiency.
Fallbrook maintain a NuVinci forum here:http://nuvinci.informe.com/forum/
Also of note to L, the previous correspondent: It seems there is also a range of "acceptable" gearing combinations with the N360 in order to avoid subjecting the mechanism to excessive torque loads, just as there is with Rohloff. Chainring-to-sprocket ratios can apparently go as low as 2.0, according to their literature. Yes, it would be best to give Robin a ring as to the particulars or float a quick check with the membership of the NuVinci forum before attempting a hybrid-drive setup to tackle the windy Fens.
The hubstripping blog mentioned above provides a wealth of information, covering a wide variety of topics related to IGH, including a flywheel KERS attached to a NuVinci hub (though how such a small mass spinning at such low RPM could provide much assistance is beyond me), a downtube-mounted Rohloff shifter, and brifters for the Alfine/Nexus. You can see it all here:http://hubstripping.wordpress.com/