I don't know much about CUDA, so I am not appropriate to answer your question.
However, AFAIK, Photoshop still mainly CPU intensive, only few filters that actually use the GPU acceleration. GPGPU may be the future, but sure not the main stream at this moment, and no need to worry about that too much (we can almost always take the same upgrade cheaper later, if really require). Also, my understanding is Photoshop itself use OpenGL and OpenCL, but not CUDA, there are some 3rd party plugin use CUDA, but it's not a requirement to utilise the Mercury Graphics Engine.
So, for many layers large Photoshop work, you better have a fast CPU (single core speed still the main key factor, but not the core numbers), and lots of RAM. For GPU, even a R9 280X may be overkill. But considering it's not that expensive, I still believe the 280X is a decent choice.
The R9 280X can pull more than 75W from the 8pin port, that mean will pull more than 75W from the 6pin source. If you mainly use it for Photoshop. There is nothing to worry about, I think you can't even warm up the card. However, for heavy video editing, OpenCL, gaming, etc. The 8pin may pull more than 120W which cause the Mac Pro activate the self shut down protection.
So far, the easiest way to avoid that should be balancing the power draw.
dual mini 6 pin -> single 8 pin -> 6+8pin.
Therefore, everything will go through the single 8pin bridge, and the power draw will be shared between the 2 mini 6pin source. The total power draw may still go above the 150W official limit under stress (which won't happen in Photoshop), but the risk to cause any real damage should be very very low. I run 2x 7950 for some time already, and slowly push both cards to higher speed to draw more power. They can now draw ~100W from each of the mini 6pin for few hours. The whole system still stable. So, I must warn you that the 8pin may overload the mini 6pin port, which may cause permanent damage. However, unless your workflow is running Furmark, there should be nothing to worry about.
