I'm having similar issues with a the Samsung PM851 M.2 2280 SSD that came with my XPS 13.  Boot from Windows USB UEFI installation media and drive is detected, perform install all OK.  However, intermittently, particularly if doing a full shutdown then restart the UEFI Firmware fails to detect the SSD (shows as "none" under system info in UEFI Firmware).  If I try to manually add a boot option I get "Filesystem not found".  Fully powering off the system and restarting a few times eventually the UEFI Firmware sees the SSD again. 

I'm running A03 of the UEFI Firmware (BIOS).

Dell - you need to fix this or I'll be returning the laptop for a refund.

I get the same link width here on A03 with the SM951. I hope that there will be an official response from Dell soon.

I realize that the best we could possibly get in the XPS 13 (after a BIOS or driver update) is x4 @ 2.0-limited 1500MB/s read & write, but that's still faster than the xp941 while using less energy due to the improved controller.

Both the chipset and physical M.2 socket support M.2 x4 @ 2.0, so why limit yourself to x2 @ 2.0 (750MB/s) when you can get so more?

I highly doubt that this will be resolved by Dell.  The chipset for this laptop only supports PCIe 2.0 and most M.2 slots only support 2 PCIe lanes.  For most PC's, you need an M.2 to PCIe 3.0 x4 add-in card to realize the full potential of that SM951, even if you have a built in M.2 slot.  I would put that SM951 in your desktop PC and go back to a fast SATA based M.2 card like a Crucial MX200.  The MX200 is faster than the PM851 the XPS 13 comes with.  It's a waste to put an SM951 in the XPS 13.

I guess my point is that even though the M.2 spec calls for access to 4 PCIe lanes, most M.2 slots on PC's only have access to 2 PCIe lanes, unless you have some of the extremely high end X99 based motherboards.  This is most likely a hardware limitation and not something Dell can fix with a BIOS update.  Either way, the thing that makes computers seem faster with SSD's is not the sequential read performance.  It is very rare that you will be accessing sequential files regularly.  Random mixed read/write speeds at a QD of 1 or so is the metric that makes your PC feel fast with an SSD.  The SM951's random 4K QD1 speeds are not much faster (and slower with small files) than the latest and greatest SATA3 M.2 SSD's like the Crucial MX200.  Samsung drives are actually known for not doing great with mixed random read/write.

The SM951's average 4K random reads (for QD1,2,4) are 50% higher than the Crucial MX200 (other low QDs are important like 2 and 4). Also the sequential speeds actually do improve performance for my workflow as a developer.

are there any news regarding the number of pcie lanes for the sm951 ? -- if a dell representative ist reading this thread:  even a negative answer i.e. "the xps 13 2015 is limited to pcie 2.0 x2" would be useful - so we don t need to look around further more

I'm also running a SM951 in my new XPS 13.  I'd really appreciate a bios update that gave use to those other two pcie lanes.

My Dell XPS 13 9343 shipped with a Samsung PM851 that has two notches in it.  How are you using the Samsung XP941 and SM951 in yours?  Are you cutting a second notch in them or do they have another model number with two notches?

Bernito8: Look inside the socket and you will see that it is M-key. The PM851 has two notches to allow it to be inserted into both M-key (SATA or PCIe) and B-key (SATA-only) sockets. There is no M.2 socket that has two protrusions, only SSDs with two notches.

no change with A04 bios -- ever slower in read

will there be a chance to get x4 or should we return to bulky, non touch apple laptops

On my new Dell XPS 13 9343 I could not get even near the numbers that some are posting here (750 MB/s ...). So the investment in a SM951 that replaced the included PM841seemed wasted. 

Since the orignal poster mentioned that the A03 bios was not good and another poster stated A04 was even worse, I decided to downgrade to A02 (it works with the SM951). This improved the measured performance significantly. It is still not as great as it could be, but I am happy enough.

I had not tested my PM841 256gb with AS SSD, so I compared to the results on Notebookcheck who had received their XPS with a Lite-On ssd (see attachments for my results).

It is sad to see that  upgrades that should bring improvements in this case seem to lower the value of the machine.

We know the XPS 13 BIOS is quite limited however there is more granular control using DELL's command centre utility. For example, folk who cannot get their Intel Smart Connect to work can re-enable it within this utility. Since you already have the Samsung 951 your in a goid position to try out some of the hidden BIOS settings.

Bump... please Dell respond!