One area where I think a better implementation of GPU processing would be extremely valuable is in responsiveness. I'm currently using an old MacPro with a basic graphics card. There are certain tools, liquefy for example, that I am hesitant to use because they are so processor intensive that there is a time lag between the stroke of the stylus and the effect displaying on screen. This creates a problem where I draw on screen and I'm not really sure what effect I'm having as I'm drawing. It makes the effect harder to use.
I would hope that the use of the GPU would make the effect responsive to a point that there was no lag between moving the stylus and warping the image.
At any rate, I think that's where GPU processing would have a distinct value.
From Adobe:
"The Photoshop CC 14.2 update provides GPU compatibility support for the new Apple Mac Pro. The 14.2 update will allow Photoshop to take full advantage of one of the two available GPUs at a time.
We expect to add support for both of the new Apple Mac Pro’s GPUs, and continue to optimize our customers experience by supporting additional GPU operations in a future release.
Each of the two GPUs in the base model of Mac Pro are faster than the GPUs available in any other Mac model, and provide plenty of speed and VRAM (memory on the GPU card) for most operations. If you spend a great deal of time editing very large images using one of the GPU-accelerated Photoshop features such as Blur Gallery, Liquify, or 3D, we expect this gain to be significant. (The 6GB VRAM in the highest end GPU option would only be needed for the largest 3D models)."
I can attest to the fact that there is no lag with the liquify filter on the new MacPro with the AMD FirePro D500 GPU, which has 3GB VRAM. Photoshop CC is still only using one of the two installed GPUs but I believe OSX (10.9.1) is better utilizing the capability of dual GPUs. Only one GPU is dedicated to running monitors while the other is fully available to programs.