All the addon cards I've seen are PCIe-2.0 x4 cards, thats 2GB/sec each direction. A single thunderbolt 2 ports would need ~2.5GB/sec each direction to 'max out', a pair would need 5GB/sec. They seem to be assuming a significant amount of the TB bandwidth will be used by the video passthrough capability. That makes these cards a not so great option if your goal is only to connect high speed external devices, not monitors.