Replacing optical drive in macbook with SSD: does it matter where the SSD drive is installed?
Yes this does matter depending on the sata speed of your hdd, ssd and the laptop.

My 2011 MacBook Pro supports 6 Gb/s to the original hdd and 3 to the Optical drive.

My ssd supports 6 Gb/s and my original hdd supports 3 gb/s.

Obviously, I did not put the ssd in the optical bay caddy (which would have been less work). Instead, I moved the original hdd to the caddy an placed the ssd where the hdd originally used to be.

This way, my ssd runs at 6gb/s and my hdd (still) at 3 Gb/s.

Good luck!