Right, I recognize that my use case is particular (driving a pile of projectors from one machine). We looked at adding additional video cards to, say, an iMac,

Right, I recognize that my use case is particular (driving a pile of projectors from one machine). We looked at adding additional video cards to, say, an iMac, but so far as I know, nobody is selling Thunderbolt graphics cards, and none of the Thunderbolt enclosures support adding graphics cards to OS X. Maybe that's changed, but I don't think so. Editing to add: and more broadly, part of the virtue of the old pro was that if you wanted to do something weird, you could open it up and shove in whatever you wanted, unlike every other product in the Apple line. It's a shame (though not unexpected) that this new iteration sacrifices as least some of that in the name of size, something I don't think most pro users really care about.