I put a 850 SSD in my 2012 13" MBP a few weeks ago. I wanted to start with a fresh copy of the OS so I simply installed the SSD, formatted it and then did the internet recovery. I made the mistake of connecting to my wifi which for some reason, internet recovery was going to take several hours to do. I then stopped the download, connected my ethernet cable and rebooted. The recovery then took minutes instead and was very fast.
Once that was done, I connected my old HD to a docking station, one that I simply plug the old HD into and copied my iTunes music from it's music folder to the SSDs music folder and all was right with the world.
I though that installing 16GB of RAM into my MBP made a huge difference but installing an SSD makes my 2012 MBP seem just about as fast as my 2015 15" MBP. I will be using the 2012 more often now. I bought both MBPs new in 2015.