Check out the Bluebird benchmarks [0], or you can easily try it for yourself - create a simple web server which does some makework async task, like stating a bunch of files, then benchmark with ab or something better.
You haven't provided any evidence so far. You owe evidence because you made a claim. If you are unsure whether Bluebird's Promise performance makes a difference in a real-world scenario, why make such a claim?
You are asking a question, I do not owe you an answer, it's up to you to find out whether this stuff matters for your use case. I've done these tests myself, it is trivial for you to do the same. Do your own homework.
> You are asking a question, I do not owe you an answer
I agree! But you did answer it:
> Bluebird is mainly designed for node (V8) and promise performance can make a massive difference on the server.
If I'm sacrificing simplicity (by using a non-native Promise implementation with a very large API) because of performance improvements, I want data (proof) of these performance improvements. If you don't have the data that's fine! No harm done, I'll keep waiting until someone does.
[0] https://github.com/petkaantonov/bluebird/blob/master/docs/do...