Hi @Meephan  welcome to this free user to user Alienware laptop discussion forum. This is not Dell Support. 

The m15 R7 System Board (aka motherboard) Chipset to CPU has PCIe bus speed of up to Gen4. But this does not reveal the answer to your question. Storage says PCIe Gen4 x4 NVMe supports speed of up to 64 Gbps (double the speed of PCIe Gen3 x4 NVMe of up to 32 Gbps). Hopefully the author's claim is accurate. 

Im about ready to add the same ssd to my M15 r7.  How did it work out?

Hi @Meephan and @rflery Samsung 980 Pro 2TB SSD has a nasty firmware glitch that could kill your SSD OS (C:) boot drive. The affected firmware is version 3B2QGXA7, upgrade to the newest firmware as soon as possible. That version would be 5B2QGXA7. Samsung SSD Magician 7.2.1.980 Download | TechSpot. 

Samsung 980 Pro has a nasty firmware glitch that could kill your SSD | TechSpot. 

Wow! Thank you so much. I have not installed it yet. I may just get a different one. Crucial P5?  Whatever is the easiest and most compatible 

Hi @rflery the Samsung 980 Pro is always in the top reviews. Everyone should use the Samsung Magician application to check and upgrade their SSD to the newest firmware version as soon as possible.

The Samsung 980 Pro 2TB has more features than the Crucial P5 Plus 2TB PCIe M.2 2280SS Gaming SSD | CT2000P5PSSD8 | Crucial.com. Laptops can go 'missing' and the Samsung drive-encryption option will prevent third party access to user's bank accounts, etc. 

I would suggest installing a 3.0 SSD instead of 4.0. I used 980pro in the past, but it does not work well. I have already replaced my 980pro with a 3.0 SSD, then everything works perfectly. The second slot of m15r7 is not compatible with a 4.0 SSD at all. 

Hi @Meephan Dell advocates that both slots are Gen4 NVMe compatible.

Users are encouraged to move the Dell supplied Gen4 NVMe boot drive to the SSD2 slot to confirm compatibility. If good, the purchased second drive may not be compatible until its firmware is updated. BIOS and Samsung Magician should see the internal second drive, even if Windows operating system's Disk Management does not. 

Get enclosure (or similar) so that external compatibility is good and upgrade firmware version so that it can be used as internal SSD2 drive. Samsung Magician will upgrade Samsung SSD drive firmware version.