McKinsey claims that on average diverse companies outperform homogenous companies http://www.mckinsey.com/business-functions/organization/our-...

With that said, their survey includes companies across all industries, including those with a lot of customer face-to-face interactions (retail, banking, airlines, hospitality, healthcare). I am not aware of anyone narrowing down the research to tech/engineering field, so in absence of that the public just assumes that similar findings would apply to every industry, and diverse high-tech companies will outperform non-diverse ones.