I'm not sure if that's true. I don't really get stressed or anxious, and from the few times I tried to put myself into stress mode I've performed much worse.

There is a great episode of Invisibilia podcast series on this subject. They talk about rare case of a person without fear and she learns how to survive without fear. https://www.npr.org/programs/invisibilia/377515477/fearless

Fear is rarely good, surely? Maybe if you potentially have to literally fight for your life, yes, but that - I hope - is a rare occurrence.

Your argument defeats itself.

You say fear is good, presumably because it stops you from doing things you don't know are dangerous.

But then you say you can do a technique to defeat fear when you know the fear is irrational.

But your argument starts from the premise that you don't know a situation is dangerous or not without the fear so how would you know it's irrational?

In my experience it's the opposite, most fear is not useful.