Ahhh just thought it was something different with command line. I've only got an I5 2500k and see a max of 6000 nonces/min with 4 threads but have never used the async option.
Nothing tricky in my .bat:
wplotgenerator 4163282010088137402 76054000 7625000 25000 10 /async
PAUSE
I think it's more just high-end (and well overclocked) cpu. Also, put in async mode, so plots are still being constantly generated, even when they're also being written to disk. With gpu plotter, there's a lot of down time on the gpu because no plots are created while they're written to disk. So ya, faster (and I imagine cheaper) plotting with cpu plotter.
Of course, the gpu plotter has capabilities to use lots of gpus and to write lots of plot files to different directories at the same time...