I'd like to upgrade my late 2013 retina Macbook Pro with a M.2 PCI-E SSD. Is this possible? Do I need an adapter?
As you can see in the image below from ifixit, the 2013 Retina Macbook Pro does have a kind of M.2 PCI-E SSD. (It is the black rectangle the person is holding.)
This Ifixit guide shows you how to install a new drive, and links to a page that lets you purchase a compatible drive.