Short Answer for transcoding 4kHD 8:1 to ProResHQ422 1080p, over any x8 lane pci-e thunderbolt expansion with a Red Rocket.
Macbook Pro 17" = 30 fps, uses half system resources
Macbook Air 13" = 18 fps, uses all system resources
For comparison an 8 core Mac Tower = 30 fps, uses half system resources (internal pci-e x8 lane slot). In the case of the Mac Tower and Macbook Pro 17", the limiting factor is the Red Rocket card itself. Next up is dual Red Rocket cards in the thunderbolt pci-e case.