I don't have studies but my experience of those who excel in leetcode is that they suck at system design and good programming practices - largely because they focused on leetcode to the exclusion of all else. They "hacked" the programming interview.

And while hacking the programming interview does demonstrate grit and persistence, it doesn't demonstrate actual programming skills. The worst part though is that it generates a mindset that "good programming skills" means "even more advanced leetcode".

I don't begrudge these people. A set of absurd incentives was put in place and they reacted to them accordingly.

It isn't good for Google either. The hires that came after this philosophy have achieved very little of note. The vast majority of post Gmail abject project failures have been built in house while the successes (maps, android) were bought.