First off, *drool* over the 8 x 960 pros lol.
What version of UEFI?
All drives on the latest FW from Samsung?
Both slots hosting the HyperX cards set to NVME raid in UEFI?
Pure UEFI environment or using CSM?
Checked the write cache settings in Device Manager?
Checked the array details in RaidXpert2 in Windows? It shows a lot more info about the array than UEFI.
Download HWiNFO64 and check that each drive is getting it's full x4 of bandwidth. Even in a RAID you should be able to see the lanes assigned to each one. Here you can see my 2 x 950 Pros in RAID 0 getting x4 each (about 4/5 of the way down the list):
These are the common things I can think of. It sounds like they're being starved for PCIe bandwidth. Below are my speeds on 2x 512GB 950 Pros on the DIMM.2: