So I finally but my 4TB WD red pro x4 raid 0 to test on this Zenith Extreme TR x399 BIOS 0801, using the mentioned raid controller on a PCIe card, CrystalDiskMark reported >800 1GB qt32t1 seq speed for empty drive so all seems well. Now with 15% in use of the HDD raid 0 the CDM measures a bit below 800 and ~670MB (windows performance monitor) when actual reading 10+5GB files while moving them to NVME 0 raid/ram disk... fully cached write of that 10GB file from ram disk to the HDD raid 0 windows file transfer speed was all over the place 2-3GB/s (I wish).
Now during boot I noticed something fishy the Marwell flash up and raid set up (bios ctrl+m) shows the PCIe link speed as 2.5Gbps x1 in x16 Gen 3/2 or the Gen 2 x4 slot. The card is PCIe gen 2 x2, so that can't be right if I can get above 500MB/s speed (no cache). Out of curiosity I plugged the PCIe add-on to my old Asus P5Q deluxe only PCIe slots (gen 2.0) and connected 2 SSD. On the Old computer the Marwell BIOS shows PCIe link speed as 2,5Gbps x2.
So if I'm not mistaken, on both MB's the Marwell BIOS shows it wrong technically, as it should show 5Gbps x2 to be correct. Then again it seems I'm getting the max 1000MB/s (8Gbps) of gen 2 x2 even if it reports 2.5Gbps x1 (gen 1 x1), if CrystalDiskMark 6.0.0 (x64) is to be trusted on HDD speed.