> hopefully you know decaf is really just half caff

In the US, to be labelled as decaf more than 99% of caffeine must be removed. The EU wording, iirc, is a bit weird but amounts to essentially the same requirement.

So decaf really is decaf. The only people who might notice some effect are those who never drink any caffeinated drinks at all, and even then I'd say they wouldn't notice much.