It's not really clear to me what that would mean. Auto-replicating customer data like docker images across national borders is a non-starter for legal compliance reasons. That's got to be an opt-in, not an opt-out. So they can't just point outbound GCR requests at the EU registry and assume the customer's images are there.
It's not a mirror, it's just another place to put your images. If you're eu based then it's definitely better to use eu.gcr.io for everything from the start.
Yep, I started copying my images to the EU and Asia regions and now it's fine cost-wise. They don't have an Australia region for the container registry though, so launching VMs there is still pricy for me.