After testing the first version of the Seagate 3TB with xbench on a 12-core Mac Pro, here are the results. It should be noted that this is the first version of the 3TB Seagate Barracuda XT and that the widely available versions due out soon(?) will likely have improved performance characteristics.
3TB Seagate Barracuda on RAID Card (single disk, not RAID 5):
1TB Western Digitial Black not on RAID: