Thunderbolt is not compatible with the tower Mac Pro. The PCIe 2.0 slots on the tower max out at 8GB/s. Thunderbolt 2 is 20GB/s. The PCIe slots cannot support the bandwidth of Thunderbolt.
However, you can get USB 3.0, USB-C and/or eSATA. We offer several options:
USB 3.0 - four ports with one controller; max total bandwidth of 5GB/s - if you connect one device, you will achieve the full speed of 5GB/s on that device. However, if you use two USB 3.0 devices at once, the speed will be divided between the two devices - each will achieve 2.5GB/s.
USB 3.0 - four ports with four controllers; max total bandwidth of 20GB/s - if you connect one device, you will achieve the full speed of 5GB/s on that device. Even if you connect four USB 3.0 devices at once, all will achieve the full USB 3.0 speed of 5GB/s.
USB 3.1 - also called USB-C is 10GB/s, double the performance of USB 3.0.
USB 3.0 (two ports) and eSATA (two ports) combo card - ideal if you have external drives with both connections.
eSATA (two ports) - if you don't need USB 3.0.
USB 3.0 is 5GB/s and eSATA is 6GB/s. However, one is faster on write speed and the other is faster on read speeds. They are both similar. If you are looking into purchasing external drive storage, go with USB 3.0; it's newer technology.