That's interesting. We did an extensive series of A/B tests a while back between "Registrieren", "Anmelden" and a specific call to action (this was a german top100 site). There was no significant difference between the former two but, as expected, a much better conversion with the specific call.

However, none of this is relevant to the "Login" vs "Sign in" debate. If that button influences anything then it would be in the retention, and I doubt the impact can be measured in insulation either way. It's merely one of the tiny details that make a difference in aggregate.