I've been using theanine for a long time, but never for any of these purported benefits. And the benefits I do use it for would be near impossible to measure. I just use it to make a over-caffinated monkey brain state tend toward a "lock-in" mental state. That's super hard to measure, and just as likely the theanine is a trigger for a mental deep dive that could just as well be sugar. But the ritual works, and that's what's important to me. It just took the intial "It helps mellow out caffeine for deep focus" idea to establish the ritual.

Science? No. Effective? Yeah, I think so.