Someone posted these efficiency test results over on wreck bicycles.tech some time ago. I seem to remember that the Rohloff did not fare as favorably against derailleur systems as some of the other gearhubs, but there are a few things to consider. The Rohloff is currently the only gearhub that is oil bath lubricated, and must necessarily use seals. In my own experience, these seals do create some drag (which I really only notice in the work stand) which does decrease with use.
Even so, if I recall correctly, the Rohloff came in somewhere around 94% efficient, which is slightly less than Rohloff's own claim of 96% efficiency. The overall efficiency will be an average over all gears, as it will vary depending on how many steps of gearing the power flow follows in any particular gear. Optimal efficiency, we would expect, should be acheived in direct gear.
Derailleur gearing efficiency will vary as well any time an absolutely straight chain line is deviated from.
Speaking as someone who does not, nor never has ridden competitively, nor do I track time/distace or anything like that, I did not notice an appreciable difference in drag with my own Rohloff over the XT/XTR drivetrain it replaced. I do however appreciate the ability to grab as many gears as I want or need, up or down, instaneously, whether the pedals are turning or not.