I use the Velocity Solo X2 and a nVidia GTX 680 without any problems.
As with all GTX 680 and other cards of the nVidia 600 series you dont have any boot screen (e.g. if you press ALT do invoke the boot selector menu). Once the system extension for the nVidia card is loaded, the display works just fine.
I had the nVidia GTX 680 already in my Mac Pro when I installed the Velocity Solo X2. After turning on the Mac, the system booted just fine from the internal HDD as before.
I cloned my boot HDD to the SSD on the Velocity Solo X2, opened the system preferences to set my boot volume to the SSD and restarted. Thats it.
From a user point of view, theres not much difference between a drive attached to the Velocity Solo X2 and a drive attached to the internal SATA connectors.