How do you account for the write rates essentially getting capped at ~113 MB/sec in the HD Tune Pro test(1) for SATA III? SATA II easily scales up to ~137 MB/sec in the same test. There is a similar cap in Asus P7P55D-E Premium USB 3.0 performance in Tom's Hardware USB 3.0 Performance tests(2)(3). There is a suspicious flat spot in write performance your own SuperSpeed USB 3.0 test(4), but that may be due to a limitation of the hard drive (this could easily be determined by removing the Seagate Barracuda 7200.12 500GB SATA 3.5" from the ASUS/Vantec USB 3.0 hard drive docking station, plugging it directly into a SATA port, and running the test again).
I suspect Asus' USB 3.0/SATA 3.0 solution for insufficient bandwidth in P55 systems as implemented in the ASUS P7P55D-E Premium has issues with write speed. Gigabyte's USB 3.0 implementation in the P55A-UD4P does not have the same problem(3)(4).
Hopefully this is something that can be ironed out in a BIOS update. Has anyone heard more on this?
(1) HD Tune Pro test http://www.legitreviews.com/article/1127/4/
(2) Tom's Hardware, Sequential Write Performance http://www.tomshardware.com/reviews/usb-3.0-performance,2490-4.html
(3) Tom's Hardware, Maximum Interface Bandwidth http://www.tomshardware.com/reviews/usb-3.0-performance,2490-5.html
(4) SuperSpeed USB 3.0 test http://www.legitreviews.com/article/1150/6/