Just did the same to my MBP 17 " 8,3. Put SATA III SSD in main bay. Optical bay may or may not support SATA III according to OWC:
"Special compatibility notes related to use of this product with 6Gb/s SSDs in the following machines:
2011 MacBook Pro 15" (Model ID MacBookPro8,2)
2011 MacBook Pro 17" (Model ID MacBookPro8,3)
OPTICAL BAY: The optical bay interface in these machines may either be SATA Revision 2.0 3Gb/s OR SATA Revision 3.0 6Gb/s. There is no way to specify/order this data interface with Apple, but you can confirm what interface/link speed version your optical bay offers via the Apple System Profiler.
Testing has demonstrated that Apple factory hardware does not reliably support a 6G (6Gb/s) Solid State Drive or Hard Disk Drive in the optical bay of 2011 MacBook Pros (ModeI ID8,1; 8,2; 8,3). If your OWC Data Doubler bundle comes with a 6G drive, you should ONLY install that drive in the main drive bay and utilize the Data Doubler to re-task your existing drive or install a new 3G SSD or HDD in the optical bay. PRE-2011 models can utilize a 6G drive in the optical bay, but will do so at a reduced 3G (3Gb/s) speed."