As I mentioned GPU has the same current problem as physics.
Memory requirements is not a GPU issue as the rendering solution uses bucket fill to render to my guess the g-buffer so the rendered bucket could be streamed to the hard drive using very little video memory.
nVidia is King of CUDA GPU rendering and this is how it works in theory.
Interesting question I guess for Epic is does lightmass use bucket fill or scan line rending for things like lightmaps. If scan line then no hope to decrease render times unless using network rendering. If bucket fill one could decrease rendering time by buying a CPU with more cores available but still not even close to the thousands of possible cores available to the GPU driven by CUDA or OpenCL.
More about CUDA.
Which puts the super computer with in a now reasonable price range of the average Indi developers budget.
So yeah the tech is there but until the tech becomes mainstream and bought off the shelf at Best Buy usability is still an issue.