Interesting. An earlier commenter running 4.5 posted lspci output (http://pastebin.com/vBu5pBq6) too. Their SSD controller looks like this:

3c:00.0 Non-Volatile memory controller: Samsung Electronics Co Ltd NVMe SSD Controller (rev 01) (prog-if 02 [NVM Express])
[...]
        DevSta: CorrErr+ UncorrErr- FatalErr+ UnsuppReq+ AuxPwr+ TransPend-
        LnkCap: Port #0, Speed 8GT/s, Width x4, ASPM L1, Exit Latency L0s <4us, L1 <64us
            ClockPM+ Surprise- LLActRep- BwNot- ASPMOptComp+
        LnkCtl: ASPM L1 Enabled; RCB 64 bytes Disabled- CommClk+
            ExtSynch- ClockPM+ AutWidDis- BWInt- AutBWInt-
[...]

While your controller looks like this:

3c:00.0 Non-Volatile memory controller: Samsung Electronics Co Ltd NVMe SSD Controller (rev 01) (prog-if 02 [NVM Express])
[...]

        DevSta: CorrErr- UncorrErr- FatalErr- UnsuppReq- AuxPwr+ TransPend-
        LnkCap: Port #0, Speed 8GT/s, Width x4, ASPM L1, Exit Latency L0s <4us, L1 <64us
            ClockPM+ Surprise- LLActRep- BwNot- ASPMOptComp+
        LnkCtl: ASPM Disabled; RCB 64 bytes Disabled- CommClk+
            ExtSynch- ClockPM+ AutWidDis- BWInt- AutBWInt-
[...]

Note "ASPM L1 Enabled" vs "ASPM Disabled". There are also some other differences. Is your XPS 13 also a 9350?