There is no way to use Thunderbolt with a Mac Pro. There are PCI-e USB 3 cards for the Mac Pro, such as from CalDigital, but read closely about drivers.
You do';t have to buy FW800 drives from WD. You can buy the bare SATA drives then install them in FW800 enclosures from other companies. If the drives are larger than 2.2 TB, make sure the enclosure supports them,
For fast external storage you can use PCI-e cards to connect to external drive enclosures. There are simple cards, such as
or more elaborate ones, with features such as RAID, eight SATA ports, support for enclosures with port multipliers, etc.
can support twenty SATA drives (four port multiplier enclosures with five drives each) for up to 80 TB.
You can use up to three cards.