ASUS P7P55D-E Pro
At the start ASUS launched five motherboards with USB 3.0 and SATA 6G. All of the boards were advertised under the title of "True USB 3.0 and SATA 6G" and four products used the PLX bridge that use 4x PCIe 1 lanes to interface with SATA 6G and USB 3.0 controllers from Marvell and NEC.
As you can see from the image above, the hype surrounding ASUS' PLX method has been very high and it would appear that the PLX chip has been the great savior to next generation high bandwidth technologies, but it is still following the data path that was used by USB 2.0 and SATA 3G, through the P55 chipset. The processor and the P55 chipset are linked together via the DMI, a 2GB/s link that is shared by everything other than the single or dual PCIe 2.0 16x slots generally reserved for video cards. Since the LGA 1156 processors only have a single 16 PCIe 2.0 lane, if you use two video cards the bandwidth is automatically put down to 8 lanes for each slot. With a single video card you keep all 16 PCIe 2.0 lanes for the video card.
In theory, it may be possible to saturate the 2GB/s DMI link under heavy use. With just about everything sending data through the DMI link, I could see heavy usage of audio, file transfers, network data and such starting to saturate the available bandwidth. In my limited testing with the ASUS P7P55D-E Pro it didn't happen under what I would call normal conditions, but it is a possibility.
It should also be noted that ASUS did not add RAID support in SATA 6G to their lineup of P55 motherboards. They do offer an add-on card called the ASUS U3S6 that adds two more additional USB 3.0 ports and two more SATA 6G connectors.