If I understand what you're getting at, you mean something like a sub-forum. They did a private beta as well as internal tests before the site launched for UI issues, and presumably tagging vs. sub-fora came up then, or during design. Of course, 4 years of activity is its own test, demonstrating on the concept works reasonably well.
However, I don't recall anything other than that initial user testing, like in the form of studies or anything like that. Joel Spolsky put it best as to why they used it in his launch blog post:
We have tags. Every question is tagged so, for example, if you’re a Ruby guru, you can ignore everything but Ruby and just treat Stack Overflow as a great Ruby Q&A site. A single question can have multiple tags, so you don’t have to figure out which single category it fits
That aside, Stack Exchange does have a form of static categorization with tags. For example, if you go to mysql, you'll get:
And of course, there's the Facebook Stack Overflow example, which is even closer to the idea of "static thematic sections", but turned out to be somewhat of a failure. The difference is that the categorization isn't determined from the top-down, but by the users themselves in, as Jeff Atwood put it, a "folksonomy".