As promised, AMD just rolled out an update that delivers NVMe RAID support for the X399 chipset. This upgrade can be downloaded for free, you can find the instructions over here.
AMD says you can expect impressive performance scaling:
In our own performance testing, we’ve been seeing some blistering results from our test systems—a monstrous 21.2GB/s from six disks in RAID0! But RAID users know that scaling matters, too, and X399 NVMe RAID still looked great in our lab: 6.00X read scaling, and 5.38X write scaling, from one to six disks (see chart below). Performance will naturally vary based on the model and quantity of SSDs you use, plus the test pattern of your benchmark, but it’s clear that our free NVMe RAID solution can scale and scale fast.