We wouldn't care whether the user became aware of it being a shared site, as they'd know about the superentity prior to being aware of subsets.
Easier to explain with examples.
www.acuraworld.com --> this is the superentity, and all that exists right now. It's getting terribly cluttered, so next year we're hoping to launch each of the models as their own subdomain (tsx.acuraworld.com etc), and leave the existing metadata and integrated sales/off-topic intact, while allowing each model to have its own complete community and regional meets/clubs.
Going to be a massive project, and I've been trying to work it all out on paper. I'm glad someone else is doing this same kind of thing already - I'll have to pick your brain!