Partitions IMO bad Idea on real disk for would cause excessive arm movement. There are no arm with SSD just use it a single drive for everything. If you can add a second SSD if you have then on a raid controller raid 0 may increase performance. If you want to get into video consider a Windows workstation they can be configured much better then MacPro.... Unless your tied to Finalcut for you would need to switch to Adobe Premier Pro and After Effects.
I also no nothing about thunderbolt performance
A, B, and C are monumentally bad ideas.
That leaves D as the obvious best alternative.
The optimal solution is a second, physically separated, dedicated internal drive for Photoshop scratch.
However, external FireWire drives for that purpose have worked exceedingly well for me over the years. Thunderbolt should do even better.
I had procrastinated looking up the Trash-Can Mac Pro specs. I cannot believe they would consider a machine with a single HD! As far as I' concerned, after 29 years of using Macs, now the new Mac Pro is in the trash can.
I don't know why A would be a bad idea on a Mac. Its ideal for a windows system a 1TB raid 0 made of SSD works great just ask Noel. I would think it would be great for unix as well Apple seems to think so to For their MacPro storage looks like raid 0 to meStorage, User accessible
PCIe-based flash storage
Configurable to 512GB or 1TB2