Steve Sailer is one of the best bloggers around. But I'm not sure he's right here:
I suspect that it's more likely that ethnic diversity stifles innovative thinking by making political correctness more mandatory to keep the peace.
There is, however, some evidence from company performance which suggests that diversity can have beneficial consequences. This paper says:
After controlling for size, industry, and other corporate governance measures, we find significant positive relationships between the fraction of women or minorities on the board and firm value.
Diversity, it seems, is associated with some sort of superior performance - be it outright innovation, creative thinking, less bad management or a willingness to see talent wherever it exists. Whatever it is, this is evidence against the view that diversity necessarily stifles good thinking.
This, however, just deepens the question Steve raises. Why is it that Britain had far more great creative intellectuals 150 years ago than it has today?