I think there are three potential issues with this, firstly, minorities are being discriminated against and paid less in the scenario so it wouldn't be desirable on that metric. Secondly competition in many markets isn't robust enough to have an even playing field if you wanted to start a new company with all "minorities". Third, if the company was all "minorities" you would be drastically reducing the hiring pool so even with good competition it would be hard to hire well (which could wipe out any minor efficiency gains).