Hm, I missed that link to the chart before, and I was trying to respond to the "cognitive diversity -> effectiveness" discussion. You're right: it's hard to see the chart supporting my claim. (There are some hints at a sweet spot there in the middle, except for that drop at 50/50!)
Link to chart: http://hbr.org/2011/06/defend-your-research-what-makes-a-tea...