Frank Berto and Chester Kyle published a comparison of IGH and derailleur drivetrains some years ago in the IHPVA Journal. Also some data available on both in the third edition of "Bicycling Science" published by MIT.

IGHs, not even broken in, were a few percentage points less efficient than derailleur gearing overall. Derailleurs were also much less efficient than the frequently claimed 98 to 99% efficiency level in many gear combinations and at average input power levels. Both increased in efficiency at higher power and torque levels. Poorest derailleur efficiency was when using the smaller freehub sprockets.

Rohloff engineers also published test results in the IHPVA journal and critiqued the Kyle and Berto testing. Both interesting articles IMO. Links to both are available to members of the Yahoo Geared Hub Bikes group linked to in my signature block.

IMO both derailleurs and IGH drivetrains have their places in bicycling. Each has their own advantages and disadvantages as well as strengths and weaknesses.