Supposedly (and I have yet to see this consistently work in action) the efficiency comes from collaboration that naturally happens when there are no physical barriers.
What I generally see is I find out more about my co-workers lives and their interests than the increase in productivity and innovation from the team.
Bear in mind that the giants of our industry are built on open floor plans. So, in "real world" numbers they do hold up fairly well.
I fully accept that there are confounding variables. But, I can't help but also accept that open plans are not the doomsday device that they are often painted as. They are yet another of the many variables that go into how a company is running.
What I generally see is I find out more about my co-workers lives and their interests than the increase in productivity and innovation from the team.