I think this is a bit backwards. Diversity on its own doesn't do much to improve performance, but high-performing teams tend to be diverse. That's what happens when the hiring criteria is focused around a candidate's skill and fitness rather than their minority status. In this situation, the resulting demographic make-up of the company will tend towards the make-up of the society the company draws workers from.
Now consider someone like Google or Microsoft: Big enough to have the luxury of engaging with candidates from all around the world. They too want the very best in their team no matter their visa status, and they have the power to make it so. When you have a company which values skill above all else and has the ability to reach out of the local hiring pool, you end up with a more diverse roster.
Diversity just for its own sake can actually hurt a company. Consider a hiring quota mandating a 50-50 representation of demographics A and B, where A is dominant in the field and locale. Since you want to end up with more of B, you will lower the bar for them. You now have people working for you who shouldn't be, according to the skill-based priority mandate.
Employment is a zero-sum game, which means that when you lower the bar for B, you raise it for A. What you end up with is a 50-50 representation of both, but with A being a lot more skilled in the same field. Assuming a meritocracy (accomplishment-based promotion scheme), more people from A will tend to get promoted to higher rank. Someone will notice the disparity and, like the good-hearted people that they are, will rush to heal the injustice, by (what else?) mandating quotas for the higher ranks. Having prioritised properties over qualities, you end up with a weak leadership.
To answer your question:
[What are the ] benefits of multicultural environments compared to mono-cultural workplaces
I have no studies to back any of this up, but I can run the same thought-experiment with a hiring demographic mandate for A. First, let's assume that your workers are blind to this. At T=0, your 100% A workers form a nice bell-curve over their skill level. You take the good with the bad, everything evens out. As time progresses, skilful people (those with a passion) will want to challenge themselves and go higher, and those in lower skill levels will want to become entrenched to achieve job security. From this point on, your organisation will tend towards toxicity, which will turn away any skilled pigeon that happens to fly in the hole.