As several other posters have mentioned, there's plenty of anecdotal evidence that XP (and XP-like practices) have improved efficency.
Just in the last couple of weeks, I was adding some loosely defined behavior to a system. I knew what it had to do, but I didn't know how to implement it. I wrote a handful of unit tests first, which forced me to think about the API I was about to create. It took maybe 20 minutes to get the tests into shape.
Fifteen minutes after that, they all passed. Writing and debugging the actual code was extremely easy, once I had test in place.
If I'd had a better spec in place, it's entirely possible that I could have written the code without the tests -- but I'm almost never that lucky.
Maybe there are development shops out there that don't need to read 'The Mythical Man Month' and 'Peopleware' over and over again. For the rest of us, I'm pretty happy someone's pointed out that combining several of the practices everyone has known are worth doing for years has unintended and beneficial consequences.