Apple has released iMac EFI Update 1.6 that reported unlocks the full 6Gb/s SATA 3.0 capabilities of two of 2011 iMac's internal drive bays, according to OWC.
While iMac EFI Update 1.6 is described as including "fixes that improve performance and stability for Thunderbolt," it would also seem that an unadvertised benefit is that it also unlocks the full 6Gb/s, SATA 3.0 capabilities of two of the internal drive bays.
OWC says they will be doing some testing and will post number to show the speed differences. Unfortunately, the optical bay remains at 3Gb/s.
Like as if there is a chance that it will ever go that fast.....Even if you have 4 WD Raptors @ 15K in a RAID0 config you won't get that speed so how do they expect to get it from any hard drive that you can put in the iMac-solid state drives don't read or write at that speed...