I also found an interesting recent article by Barry Boehm on “Managing Software Productivity and Reuse” [pdf], that details the results of an extensive analysis conducted with the DOD to discover savings over a business-as-normal approach.
In this study, they discovered that you could achieve an 8% improvement through “working harder”, a 17% improvement through “working [...]

I’ve been trying to find more information on the “factor of 10″ productivity differences, between either teams or individuals, frequently cited, but most of the primary articles don’t seem to be available on-line. A trip to the library is probably in order again early next week.
I did come across this study from 1995, however, which [...]

Project managers cannot afford the time to be entirely scientific when
studying failure data. In order for them to control software processes,
they must proportion their responses according to the significance
of the data for the success of their projects.

— Jerry Weinberg, Quality Software
Management Vol 2, Chapter 3