I've already written about how I don't find clipless pedals improve my performance enough to be "worth it" for me somewhere in the middle of this thread.

As to efficiency that's hard to say, and I'm not convinced that is the most important criteria. It depends what you mean by efficient. If you mean wasting less energy when spinning high rpms, then probably clipless pedals are "more efficient". On the other hand considering efficiency from a purely mechanical point of view, spinning at higher rpm should be less efficient, because for a given power output, you are wasting extra energy accelerating your legs. However it is clear that mashing hard is going to tire out one's leg muscles, so maybe it's that spinning results in better performance, though with greater energy use.

I don't have a power-meter and I haven't timed myself with and without clipless on the same bike in quick succession, so I can't say what the difference in performance is for me. I would be interested if others could post such results.