OWC has updated its SSD page, saying that its PCI Express SSD card is coming soon. Their claim: "Get faster than 6G SATA speeds in your Mac Pro or PC desktop/tower." According to wikipedia, PCIe 2.0 allows up to 5GT/s (Gigatransfers per second) and 500 MB/s per lane. According to everymac.com, on the Mac Pro "Six Core" 3.33 (2010/Westmere):
I assume the speeds are therefore achieved by using multiple Aura Pro Express cards in raid 0 format, run through the PCIe 2.0 slot.This model has four full-length PCI Express (PCIe) 2.0 expansion slots, two x16 slots and two x4 slots. In the default configuration, one PCIe 2.0 x16 slot is occupied by the graphics card. Apple also reports that "all slots provide mechanical support for 16-lane cards" and there is a "300W combined maximum for all PCI Express slots."