Evidence that open source production processes increase efficiency and/or consumer surplus?
There is a large economic literature on intellectual property rights. However, the issue seems far from settled on what even the optimal duration for patents are. Note that open source is even a step further than a 0 day patent duration. A strong case for your view would probably be found in Boldrin/Levine:
http://levine.sscnet.ucla.edu/general/intellectual/againstnew.htm

Here are several further starting points in the literature:

Scotchmer, Suzanne. "Standing on the shoulders of giants: cumulative research and the patent law." The Journal of Economic Perspectives (1991): 29-41.

Besen, Stanley M., and Leo J. Raskind. "An introduction to the law and economics of intellectual property." The Journal of Economic Perspectives (1991): 3-27.

Posner, Richard A. "Intellectual property: The law and economics approach." Journal of Economic Perspectives (2005): 57-73.