I'm going to give you some of what you're looking for. I did a lead architect stint at an ASP, and chose ACS as the platform. There's a little write-upon my
case studies page.
It was wonderfully easy to integrate the domain-specific data model with ACS's, and I don't remember differentiating site look and feel being a very big deal with the templating mechanism.
We didn't get into production, so I can't speak to that. But throughout the 4 month development cycle the team never hit a gotcha that suggested we made a bad platform decision.
I would think that Greenpeace's experience (with many localized subsites etc.) would also imply that you could succeed with you plan.
