The huge padding is the thing I hate the most about Gnome, and if Cosmic will have the same issue, well, then that's a bammer.
With Gnome, however, there are extensions and themes that allow to alleviate the problem, even if partially. And I don't think Cosmic will have any of those on day one (if they there's no customization built-in, that is).
I am not sure if they have reach that goal, but that is their goal.
With Gnome, they are very willing to break all plugins at all times. They simply do not care for stability of plugins, its a nonissue for them.
In Gnome plugins are JS application all running in the same namespace, a single crash of those can take down everything. In Cosmic these are Rust application running in different namespaces.