[cmsj]

you could make a pretty safe argument that the iMac Pro is right up there with the best gaming PCs one could buy/assemble.

[opencl]

It really isn't. The fastest GPU available is a Vega 64 underclocked to basically the performance of a normal Vega 56. A 1080Ti is ~50% faster. Even if you connect an external 1080Ti it's constrained by TB3 bandwidth.