Is Mid-2015 Retina SSD compatible with my mac?

While Apple is doing their own thing here lets see if we can leverage the M.2 standard here for clarity. Please review this doc: Wikipedia - M.2 Standard. Here we can see the keyway of the M.2 card & slot defines the interface dialog. In the case of M.2 it would be A for PCIe x2 & M for PCIe x4. So in the case of M.2 standard one can't insert just any blade drive in as the location of the keyway needs to fit.

OK, lets look at Apple's SSD blades. Apple has used so far four different SSD's. In addition the SSD's firmware has also been altered so even with the correct physical SSD it won't work in all the systems that support the same physical connector. So you need to use the correct part for the given system for the older systems (mSATA, SATA based SSD's).

Now lets jump to the last few years when Apple appears to have settled down a bit. Today Apple is supporting both PCIe x2 & x4 lane SSD's which appears to have the same physical interfaces. Apple has not disclosed what is interoperable between the series and the part numbers across the systems are unique.

If we look at the SATA interface which used a common physical interface standard there was (still is) a lot of confusion on what is interoperable. So someone with an older SATA I based system has to be careful what drive they used as the drive may not play nice in their system. If you review a lot of the questions here you'll see people encountering problems until they put in a drive that supports SATA I in these older systems.

So in this case Apple or the 3rd party needs to give us guidance on what is interoperable between Apple's custom SSD units or someone needs to be willing to do the needed testing to see what is workable. So far no one of good authority has stepped up to do this testing (nor have I).

So I can't give you an answer if the older PCIe x2 systems will support the newer PCIe x4 SSD's or if the newer PCIe x4 systems support the older PCIe x2 SSD's.

Given the cost of the SSD's and having messed up a few by putting them into the wrong system I would recommend you not try unless you are willing to take the loss (SSD or even the system).

Just like driving, its best to stay on the correct side of the road even if it looks like you can drive on the other side. Only problem here is you may end up in a nasty crash without knowing until its too late ;-}