Have not tried exactly that, but have put in the HP Texas Instruments USB 3.0 "2x2" PCIe card and it is working great.
The thing to know about the xw6600 is that it has two PCIe Generation II slots, which can run up to 5Mbps. Those are the two video slots, and the other PCIe slots are Gen I. It is not until you get to the Z620 that you get Gen III (10 Mbps) slots showing up.
So, place the card you choose into the second (lower) video slot for fastest access to the PCIe bus.
Personally, I've moved away from SAS, and don't see big differences in user experience between SATA II and III. So, for me the use of USB 3.0 with 5 Mbps speeds was the thing I really benefited from. And, I was surprised to find that my chosen (fast) USB 3.0 thumb drive actually is significantly faster than my older USB 2 thumb drives even when plugged into HP USB 2.0 ports. Connected to that HP USB 3 card it is amazingly fast. I've posted on that card and how I use an Akasa front 3.5" form factor interface to get 2 of the 4 total USB3 ports up front where the floppy used to live. Very nice clean install.