The OWC Mercury Elite Pro Dual houses two hard drives in a high-grade aluminum case and offers Thunderbolt + USB3 connectivity along with user-selectable RAID modes or independent drives, and is bootable.
RAID made easy: no technical knowledge required.
Thunderbolt vs USB3
The graph shows that USB3 can handle dual fast hard drives very well, certainly no less well than Thunderbolt. This graph is for DiskTester fill-volume on the first 1TB of a 6TB RAID-0 stripe of Toshiba 3TB hard drives.
Note that Thunderbolt writes are actually less consistent than USB3. The test was run twice and this is actually a little better for TB than the other run. But it should have no real world bearing.
USB logic steps:
Drive > SATA to USB controller > USB cable > USB host/chipset
TB logic steps:
Drive > SATA to PCIe controller > TB controller > TB cable logic > cable > TB cable logic > TB Host controller