The OP replaced the HD cable, not the optical bay cable.
There are known issues getting 6gbs with optical SATA connection in the 2011 MBP.
MCE has a product, "OptiBay Extreme" which says will give 6gbs to certain 2011 MBP's. The OptiBay Extreme is $99.
http://store.mcetech.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=OBSXGB-XTRM
This web page has the following:
VERIFY: To check if your Early or Late MacBook Pro can benefit from the OptiBay Extreme, hold down the option key on your keyboard while going to the Apple Menu at the upper left of your screen and select the first item there which should say "System Information". Then select, depending on your Mac OS, either "Serial-ATA" or "SATA/SATA Express" and there you will see on the right pane two Intel 6 Series Chipsets listed. While the top chipset normally corresponds to the main drive bay and the lower one to the optical drive bay check to see that in the information below that the "Link Speed" for both chipsets indicates "6 Gigabit." If so, then you can use the OptiBay Extreme to get full SATA-III (6Gbps) speeds when using a SATA-III SSD, Hard Drive, or Hybrid Drive in that location.
So basically, MCE is saying if you don't see a 6gbs link speed, you have no chance of getting 6gbs on the optical bay on your MBP.
You don't mention why you need the "cushion" for the HDD in the [corrected] optical slot. There are 7mm 1TB HDD's and 9mm 2TB. I don't know if you can get 7200rpm with a 7mm 1TB though. If you're using a cheap bracket for the optical bay and it passes the test for link speed above, maybe a more expensive bracket (from OWC, for example) may give you 6gbs but I think the chances of that happening are low.