The second chipset (the SATA -> USB one) in their RAID enclosure in an
Oxford 3100, which only supports SATA II.
I've been looking over the Oyen MiniPro series of enclosures carefully, and it appears that getting Firewire with your enclosure is a limiting factor, because the USB chipset included with all of the Firewire-compatible packages only supports SATA II. The only models with pure USB 3.0/SATA III use the ASMedia 1053e controller, and they are the USB 3.0 only enclosure and the USB 3.0/eSATA enclosure.
----------
I found
this article very helpful in explaining why USB 3.0 often does not produce the speeds we expect. UASP is explained.