What speed is the SATA controller in a July 2012 macbook pro?
OWC has tested it and confirms that..


  ...the MacBook Pro 15” (2012) has two drive bays, each capable of running an OWC 6G SSD at full 6Gb/s speeds.


which is the SATA Revision 3.0 speed.

They've also setup a striped RAID which benchmarked at "over 1000MB/s read speeds and write speeds that nearly hit 900MB/s".

Regarding the Macbook Pro 13" (2012), OWC recommends using a 6Gb/s SSD only in the main bay, as the 6Gb/s performance of the optical bay is very poor. 3Gb/s models however work without problems in the optical bay.