I've also seen 20% improvement with TypeScript when working with developers in a big corporate environment (especially when no one cares about the project and everyone gets paid hourly).

But when working with senior developers on open source projects (when people don't get paid), I've seen at least 50% drop in developer velocity using TypeScript and lower code quality.

My theory is that developers who are passionate about coding don't need TypeScript to produce high quality code. It mostly just takes the fun out of it.

But I guess if there is no passion to begin with, then I can see how TypeScript can't hurt in that respect. I've worked for a company like this once for 6 months; I had to quit because I enjoy coding too much to spend most of my time writing and renaming type definitions and waiting for the compiler.