Jonathon N. Cummings, an assistant professor at the MIT Sloan School of Management, has done some research on the impact of structural diversity on project success, as reported in Sloan Management Review's Winter 2004 edition: Building Better Teams
There were two major findings. First, teams that shared knowledge, both intragroup and externally, tended to perform better. This result confirmed much earlier research. Second, as the diversity of teams increased so did the correlation between external knowledge sharing and performance. That is, structurally diverse teams did not necessarily perform better (or worse) than their homogeneous counterparts. But structurally diverse groups did appear to be better equipped to take advantage of knowledge shared with outsiders.
Obviously, I like to hear the first conclusion. It justifies my existence as a knowledge management consultant. The second finding brings together a slightly different facet. While structural diversity isn't a guarantee of better performance, it does have some correlation with better knowledge sharing -- and the sharing leads to better performance. (Note that structurally diverse teams in this study are those comprised of people from several physical, hierachical and departmental locations throughout the organization.)
A full article on this research is in the March 2004 issue of Management Science.