I agree with @jamesh, mirrors seem to be closer to a per host solution and done through configuration.
I am a bit worried about overriding anything with “security” in its name, @jdstrand what are your thoughts?
I guess I don’t see any reason why mirror selection should be tied to project description. If Snapcraft had the ability to select a local mirror, I’d want it to apply to all projects I build locally. If I had to edit the yaml or remember to always pass long command line arguments to snapcraft, I’d probably ignore the feature even though it would probably save me time.
The repositories hierarchy you’ve proposed is quite different though. It’s not just an optimisation, and really is part of a project’s configuration: the project will likely fail to build if you omit the PPA, and other projects might misbuild if you injected the PPA and they weren’t expecting it.
I agree with your point and I will be removing the mirrors 
Thank you for the feedback!