It's hard to say if the new Mac Pro signifies some sort of revival of "workstation" computers because, by many standards, it doesn't stack up as one. Expandability, often one of the cornerstones of a workstation, isn't really a strong feature of the Mac Pro.
I think it's some sort of new class. A high-power compact desktop or something.
I was trying to match the marketed spec as closely as possible. At this stage there seems to only be one board (that I could find) that supports the E5-1620v2 with ECC. It lacks USB3, has only three PCIe slots and generally isn't up to spec.
Building another high-end PC (i7-based) to out perform the Mac Pro is a whole different excercise, and I'm fairly sure it would be quite a bit cheaper.
The whole post was really just a mental exercise. Clearly it's not possible to build something that matches the Mac Pro exactly - even if you could perfectly match all the advertised specs, the Mac is on a custom board, with customised hardware and included hardware-specific OS optimizations.
Those GPUs specifically were chosen because they seem the closest match - while there is apparently a 2GB version of the card, it doesn't seem to be widely available.
I'm sure someone with a lot more disposable income than me will see what it takes to out benchmark the new Mac Pro when it ships. My guess is it will be quite a bit cheaper (around $2k) and i7-based.
I think it's some sort of new class. A high-power compact desktop or something.