The OWC Mercury Accelsior E2 PCIe SSD is a blazingly fast SSD that installs in a slot of the Mac Pro, or in an external Helios case on any Mac with Thunderbolt.
The Accelsior E2 has a bonus feature: two eSATA 6G ports. Which begs the question:
How fast are the Accelsior E2 eSATA 6G ports ?
To wring the most performance out of a regular SATA SSD on a Mac Pro, install the Accelsior E2 PCIe SSD, and use its eSATA ports for external SSDs, avoiding the internal Mac Pro SATA ports.