Most modern drives should be faster on usb 3 just because data can be sent full duplex instead of the half as it was in usb 2. Performance should basically be the same as if it was a sata connection.
Same as SATA? Not a chance. Not until USB starts adding DMA instead of doing everything the slow way by dumping everything on the CPU via interrupts.
SATA is fast because it uses its own DMA-enabled processor. Firewire is fast because it uses its own DMA-enabled processor. Soundcards are latency-free because they use DMA-enabled processors.
Until USB decides to join the world of DMA, it's more like the red-headed stepchild of transfer technologies. Cheap, but also unreliable for anything more than hooking up a keyboard, mouse and syncing an iPod.