Relating to your aside, correlation does not imply causation. Is it the diversity making those companies perform better, or better performance leading to the economic resources to pursue diversity programs?

Jonathon Haidt's research shows there are trade-offs between mono-cultures and diverse cultures. Neither is "better" than the other. Each have different traits. As a quick example, diverse groups (on average) are more creative at problem solving than mono-cultures, while mono-cultures (on average) have higher "Moral Cultural Capital" and are thus better at group allegiance and out competing other groups when the tasks are known.

An example from the real world might be that the United States has a more diverse culture and has a lot of creative output in its economy, while China or Japan are mono-cultures and have less creativity but are out competing others when the task undertaken is well known (ie not requiring creative problem solving).