Separately, the iMac Pro can now take up to 512GB memory.
But this looks like a no-brainer: twice the storage at twice the speed for about the same cost!
Apple upcharges $2600 for an 8TB SSD, but the 8TB OWC PCIe SSD is only $1600 and nearly twice as fast (according to OWC) for $1000 less money!
With two 8TB PCIe SSD cards set up as a RAID-0, the speed should be 3X to 4X the speed of the Apple SSD. Somewhere around 9 to 12 GB/sec, my guess is ~10 GB/sec real world.
Read more in my post over at MacPerformanceGuide.com.