"Efficiency" seems like it should be easy to measure. Power out ÷ Power in. Obviously nothing in life is so easy.
Here's a blog post by a randonneur who has compared power output on his recumbent to that on his diamond-frame bike (there are some related posts at the end that are also interesting).
He's found that his peak power output is higher on his DF bike, but his calories per mile and his watts per mph are lower on his recumbent. So those are a couple of non-obvious ways to define efficiency that show the recumbent being more efficient.
The author also writes about feeling "muscle-limited" on recumbents due to his inability to recruit as many different muscle groups, and "lung-limited" on DF bikes. And there's some interesting discussion in the comments.