I expect nobody outside of Google has ever truly taken the time to study it. When was the last time you saw a programmer actually research the effectiveness of their tools and not just land on "I like this. It feels right." I never have!

But I'm not sure it matters. Go was created to test the theory, not because a theory was proven. It didn't have to be successful. It may be that the studies didn't happen even within Google, although that is our greatest chance. We do know Google actually cares about data, unlike programmers.

That said, since Go was released it seems every other language has tried to copy it with their own twist, so while that may not come from a place of evidence, it would appear that the feeling of increased productivity[1] was felt.

[1] Or something adjacent. Focusing on engineering isn't necessarily about productivity. You can't discount productivity, but it is not the top engineering concern, especially in a place like Google.