.... It's too bad the dual workstation graphics cards are the price of a single socket Xeon.
On which alternative universe?
E5 1620 --> $294
E5 1650 --> $583
E5 1660 --> $1080
The v2 versions of those are going to be about the same price.
Apple is charging $250 for a 1GB AMD 5770. A workstation card with 4-6GB of VRAM would likely to be in the $400+ zone. (AMD FirePro 7000 w/ 4GB VRAM goes for ~$699. If Apple wrangles a $200 discount out of that the price that would be $499) So two would be minimally be $800+ (and more likely around $1000+). While $800 is lower than the 1660 it is definitely lower the CPUs that are probably the "good" and "better" standard new Mac Pro configurations.
But it also a bit myopic to cast this as purely only being graphics cards. If all ever push them on a Mac Pro is simply graphics then the two card set up doesn't have much value. There little to no reason why LR can't leverage the cards as computations devices though. OpenCL is portable and fits in with Adobe's need for a cross-platform infrastructure to layer features on top of.
The iMac and MBP 15" will get a roughly similar percentage performance bump since they too can leverage OpenCL.
An operative question with the 2013 Mac Pro is whether you are buying it for by primarily looking into the past and minimizing the number of applications and peripherals evolving. Or are you looking at where software and needs are going to be 2-3 years from now.