Re: it's a chimney
>>> You certainly wouldn't get better performance from internal cards ...
Is a statement about the external cards, not the bus they are connected to... though thank you for giving us the numbers because they have been confused by some people!
I'm not sure what external card would saturate the Thunderbolt 2... Rendering stuff doesn't saturate IO, because it takes much longer to render than it does to output the results. People have been using 4K transcoder cards with Thunderbolt cheerfully enough already. There is a limit to how many times faster than real-time you need to be able to shunt video around... if you can get it off your RAID very quickly at will, it kind of negates the need to bulk transfer into your machine to edit it.
Having the expensive expansion cards housed externally means they can be shared between workstations, or even taken on the road for site work with a Macbook - so a useful resource isn't tied down to one user's desk.
An external enclosure means that RAID storage can be accessed very quickly locally by Thunderbolt, but also be available to colleagues (more slowly) over LAN without your Mac having to be turned on.
Having stuff housed externally means that the Mac's PSU doesn't have to be over-specified.
Making the Mac Pro smaller and out of less material will allow it to be cheaper than the old design with the same components- savings you can use to buy enclosures. You'll still likely be left with more free desk space than at present, too.
However, for all that, I'd wait for reports of how it performs in the real-world.