I'm not sure why people are contesting the basic link between productivity and industrial clustering. It is a pretty widely accepted principal that people in "knowledge clusters" will be more productive. The bigger the cluster, the more productive the person.
The Avg. Tech Worker in SV > Avg. tech worker in Austin > Avg. tech worker in Cleveland.
http://www.citylab.com/work/2012/12/...oductive/4049/
A person doing to same job in Austin maybe just a productive as they would be in SF. But, when you scale it out and look at the aggregate employer/employee matching effects, the benefits to clustering become clearer.
If you are stating a tech company, Silicon Valley is going to have a much bigger talent pool to draw from. Conversely, when it comes times for the tech worker to look for a new job, they are going to have a lot more potential options in Silicon Valley than in Austin. The same holds with attracting investors, suppliers, and distributers. Plus, a lot of knowledge spill over happens through formal and informal personal interactions. You will obviously get some of that in Austin, but you would get even more in SF/Silicon Valley.
This also holds for other specialized industries as well. If you work in derivatives, try fining a financial derivatives networking group outside of Chicago or NY (or journalism outside NYC or DC). Some hedge funds in suburban Connecticut have been trying to locate back Manhattan because 1) it is easier to attract workers from across the NYC MSA and 2) it is easier to court big clients (it is easier to visit a midtown office building than a suburban office park). Sure you can network online, but you miss a lot of the benefits that come from personal interaction.
Sure the argument can be oversold. In theory, Philly should be more productive than Seattle. High COL places tend to price out low productivity activity (very little mfg or call centers in Manhattan) and "vibrant cities" tend to attract "higher skilled" people. But, it does seem at the end of the day, all things being equal, dense knowledge clusters will lead to greater productivity than the same employer or employee in other areas.