Yes, there's that too, I didn't mention that because the original poster has a pre-2011 MBP, but this would partially explain Hellhammer's original recommendations.
Pity you *still* can't use SATA III SSD drives in the optibay of current Macbook Pros (except it seems the 13" one), as although they're now equipped for SATA III, they don't seem to work in SATA III mode (see:
https://forums.macrumors.com/threads/1236401/ and
http://blog.macsales.com/11895-2011-macbook-pro-sata-problems-resolved):
#2 For those with a MacBook Pro 15″ or 17″ model that has SATA 3.0 6Gb/s link capability reported, it is very important to note that this EFI update does not appear to have resolved reliability of using a 6Gb/s drive in the optical bay. If you have a 6Gb/s optical drive bay connnection and are using a product like our Data Doubler, we still recommend only using a SATA 2.0 3Gb/s drive in that bay. MacBook Pro 13″ owners who find 6Gb/s links in their optical bay are not experiencing issues with 6Gb/s drives and this update doesnt affect that usage.