I can't answer for the author of the parent, but for my part I combine my own personal experiences dealing directly with employees of government agencies with the stories I hear from a friend of mine who works for a consulting firm and often deals with government employees, including a very long assignment working for FEMA as part of the Hurricane Katrina cleanup. A particularly interesting data point for me was the experience of registering a car at the Connecticut DMV vs. registering in Pennsylvania where the process is privatized.

Has anyone ever seen any scientific evidence comparing productivity of private sector vs. public sector employees?

studying the history of the civil service's relationship to elected politicians as well as hunting down first hand accounts from people who have worked in the system for years.

also the fact that Yes, Minister is regarded as the most accurate show about politics ever made. My above mentioned study seems to confirm that it is indeed scarily accurate.