Hi John,
we are a Danish company which builds professional, custom Mac Pro 5.1 with single or dual high-end GPUs, such as 1080Ti and Titan Xps.
I disagree with previous comment saying there is not much to gain.
1) the Pcie 2.0 is basically not a limit in any way for these machines, beside gaming, since there is more than enough bandwidth for anything, even on the x4 link (only 4-6% loss by putting a GPU in slot 3 and 4, and only under extreme stress)
2) the generation difference between Maxwell (9xx series) and Pascal (10×0 series) is amazing.
What is even more amazing is how the Mac Pro 6.1 cannot even compete at all, unless we sell eGPUs together with it.
As an example, a nice specced mac Por 6.1 (2013) 8 Core 3.3Ghz with D500s cannot even playback a 5K 72fps RED RAW clip.
A Mac Pro 5.1 with our modifications can run e.g. VEGA 16GB or 1080Ti, can play an 8K 48fps RED RAW clip at 24fps just fine.
Check us out at
https://www.biglittlefrank.dk
facebook.com/biglittlefrank
instagram.com/biglittlefrank
And if you need any help, drop us a message ☺
Have a nice day
Gianluca
// BIG LITTLE FRANK
https://www.biglittlefrank.dk