With the release of Adobe Photoshop CC v14.2 as of Jan 15, 2014 we now have an existence proof of the huge potential for speedup using the GPUs. If more functions follow this path, this could be the biggest gain in performance in a decade or more.
But it’s not just the 2013 Mac Pro that’s a winner; the 2010 Mac Pro sees similar gains, at least with the NVidia 5870 video card, which has dual GPUs on board.
But are both GPUs being used, or just one? Not yet clear.
Nice job, Adobe!
Update March 7 2014: this improvement has apparently been reversed—OS X 10.9.2 and Photoshop CC 14.2.1 is about 3X slower (with GPU) than shown here, yet CPU-only test is identical.