Well, the current Mac Pro (aka the trashcan) has languished a bit and suffers from the inability to swap out GPU's. The next Mac Pro which most expect this year will likely eliminate this issue, and give us more choices in that regard. Until then, the best you could do is maybe an iMac Pro running an eGPU setup over TB3. In that case, I would go with a high-end NVIDIA GTX card. Since the underlying engine of XPression is basically tapping the same sort of resources a video game would, the key is choosing a card which is well suited for gaming.
I also wonder if maybe you'd get better performance if you made a bootcamp partition to run Windows natively on the hardware, eliminating any emulation bottlenecks.
I'm a mac guy as well, FWIW, but here the shop I spend most of my time at is PC so I haven't done much experimentation with your type of setup.
#XPression