Thunderbolt->PCI-E enclosures are interesting, but not supported.
Two points, your 3,1 isn’t taking full advantage of the Nvidia GTX 570 - that’s a known deficiency in the 3,1 model which I believe only manifests under OS X. That’s why the Nvidia 680 Mac Edition and AMD 7950 Mac Edition were marketed for 4,1 models or later - the 3,1 isn’t able to feed them data fast enough under OS X.
Second, there’s a further OpenGL/driver bug we are investigating that affects faster Nvidia GPUs in 4,1 & 5,1 Mac Pros and prevents them from running as fast under OS X as they do under Windows. This doesn’t seem to affect the mobility GPUs in MacBook Pros, or iMacs AFAIK, since they run the OpenGL 3.3 renderer (command-line: -opengl) approximately as fast in either operating system.
By contrast the ATi Radeon 5**0 cards in the 4,1/5,1 Mac Pros or the AMD FirePro D*00 in the 6,1 work similarly well across OS X and Windows.