The specifications manual indicates support for X4 drives.

https://www.dell.com/support/home/en-us/product-support/product/alienware-x17-r1-laptop/docs

Gen3 is X4 which i already knew . i wanted to know if its Gen4

is that 2tb max over both slots or 2tb per slot . if its NOT the latter its not very good as i would want at least 4tb in raid0 . also gutted there is no 2.5HDD slot for storage .

thanks anyway .. will look at alternative laptops as this does not have the space i need.

Hi @JohnW73  thank you for sharing update. 

Dell has tested Storage of 2TB M.2 PCIe Gen4 x4 NVMe card per slot, and this will be covered under Dell Warranty. However, the x17 R1 may accept more than 2TB M.2 card per slot, but unfortunately it is too new for the Crucial System Advisor Tool, etc. to provide other tested configurations. 

Putting NVMe cards in RAID 0 configuration provides no performance benefit. RAID 0 is also less reliable, and if one drive becomes unrecoverable, both have to be replaced. A good disaster Recovery Plan is also required. 

If wanting to use Hard Disk Drives because they offer superior file archive properties, use NAS (Network Attached Storage). 

Interesting...I have two NVMe in RAID 0 in my x17 and I'm getting over 10,000 read and 8000 write in CrystalDiskMark. In my case it absolutely have a performance benefit! (7000 and 6000 is what you get with single drives).