Mac – How to get SATA III on Mac Pro 5.1

Looks like a known problem with the SSD. OCZ has a firmware on their forum that will fix the issue. See the post "Changing SATA speed on your "3" series SSD." in this thread.

This portion of the instructions only applies to OCZ "3" series SSD's - Vertex3, Agility3, and Solid3 - all sizes. Some SATAII motherboards, when used with one of the above drives will downgrade the drive to SATA1 speeds. We find this mostly on Nvidia chipset motherboards for PC and Nvidia chipset based Mac's. To resolve this, we have a tool that will set the link speed to be locked at SATAII and improve performance. This can be reversed when you update your motherboard/system so you can have the full SATAIII speed again.

Installing the firmware involves burning a Linux LiveCD so I hope you have a computer with the optical drive still in.

This may also be an issue with the ICH8-M AHCI SATA interface chip. I have a late 2008 MBP that came with a 250 GB HD. I upgraded to a 640 GB and had endless beach balls. Turns out the models with a ICH8-M AHCI and a PATA superdrive interface cannot be upgraded higher than a 320 GB 7200rpm HD. Anything faster or larger will stall out as the FW for the ICH8-M AHCI cannot be upgraded beyond SATA I speed without killing the PATA interface. Some models with a ICH8-M AHCI and a SATA superdrive interface can be upgraded with a FW update released by Apple. Check for updates.