Then, go ahead and buy a PCIe SATA Gen III card, plug it in to the bottom PCIe x16 generation II PCI slot in your xw6600 and tell us all if SATA III feels faster than SATA II in the 5 Gbps PCIe Gen II slot.
It will cost you about 25.00 do do this..... you'll be able to find a good card for that price I'm sure. It would be great to include some throughput testing, but I'm personally more interested if you "feel" any difference. My guess will be no, because there are other factors in play.
Having worked extensively with the xw6400 and xw6600 and now the Z400 and the Z600 (all maxed out for speed) I can tell you that what I am seeing is exactly what I expected. As you get newer the cost starts going way up for incremental improvements, and you will benefit from fine tuning your processor to the fastest, and your memory to fully populate all channels with fastest memory (to match the speed of your fastest processor)..... all this at a significant boost in cost. Plus the cost of a faster video card to match the other improvements. There will be an added "snap".
I like the new feel, but don't like how much it costs to get there. No surprise.