As per an Business Insider article last year, approximately 73% of the hash power comes from China. Over 50% just by the top 3. If coordinated, they could cause havoc. Is that a concern to anyone?
I do not think it is not as simple as having 51% of the Hash power, and immediately causing "havoc".
The power of more than 51% of the Hash power comes from the ability to create an alternate chain of greater difficulty than the public chain.
A Secret Pool that has more Hashing Power than all the known transactions could be working on a fork in secret, and could unveil that forked blockchain at some point, declaring it to be the "one true chain". This could create double-spends, and reverse earlier transactions, and otherwise cause chaos.
But the other miners would have to accept this mysterious, much longer chain that suddenly appeared. That seems unlikely.
We know that forks and orphan blocks are part of the mining process, and maybe could even deal with a forked chain of 4 or 6 blocks. But if a mysterious blockchain suddenly appeared that was at serious odds with the public chain (dozens or hundreds of blocks long), everyone involved would very likely reject it. The rogue miners would then have invested substantial hash power, costs, and opportunity cost for a chain that ultimately may get rejected.
I'm not saying its an entirely impossible attack. But such an attack would have a substantial risk of failure. It's much easier and less risky for someone with that much hashing power to just earn an honest living through honest mining.
As long as the Chinese Miners are regularly submitting blocks to the public chain, we can be reasonably confident they are not working on another forked blockchain in secret.