SSD would scream, in theory... but you'll be limited by interface bandwidth. Now, Thunderbolt would eliminate that bottleneck handily.
Depends on the SSD. Very few can saturate the SATA 3Gb/s and I have yet to see an SSD that maxes out SATA 6Gb/s. Thunderbolt is only 10Gb/s and there are several PCIe SSDs that can deliver over 20Gb/s.
Uh...what?
Anandtech Review of OCZ Vertex 3 SSD
550MB/s. That's pretty damn close to maxing SATA III 6Gb/s.