I've seen quite a few posts in forums about some 2010 - 2011 vintage Mac PCs that use Nvidia SATA chipsets. The old driver used apparently does not recognize SATA III as a valid protocol (which the chipset cannot support anyway) when it reads the SSD's information. The driver's code then downshifts to SATA I speed, and the owners new SATA III SSD they installed is crippled at SATA I speeds. I've seen this at Intel, SanDisk, OCZ, and other forums. Same thing happens on older desktop boards using the Nvidia SATA chipsets and Nvidia driver.
The Mac owners blame the SSD for the problem, and no amount of explanation will satisfy them. Since the SATA chipset, not the SSD, negotiates the link speed, it is not the SSD's fault. OCZ and SanDisk have offered a special firmware update to "fix" this issue, which simply writes "SATA II" as the SSD's speed protocol. Of course, that converts the SSD to a SATA II drive on any SATA interface.