I Have the same basic question but with a 27" late 2013. Some comments state that the PCIe slot only exists if it was installed at the time of purchase but I am not sure I believe that.
I would like to add an SSD to the PCI express slot as my main system drive and keep the internal 1TB drive for additional storage (I already have 6 external drives connected by everything from Thunderbolt-Firewire converter and eSATA to USB 3.0 so am just not interested in adding anything but an SSD to increase system performance (preferably keeping the existing internal 1TB drive!) - Yes I have the necessary tools and am able to do the install - carefully! :)