takeshi7 :
But it is different because those M.2 slots are bottlenecked by the DMI connection on the PCH. The CPU slots don't have that issue.
The PCH bandwidth is of little importance here as once you set synthetic benchmarks aside and step into more practical matters such as application launch and task completion times, you are left with very little real-world application performance benefits despite the triple Optane setup being four times as fast as the other SSDs, which means negligible net benefits from going even further overboard.
The only time where PCH bandwidth might be a significant bottleneck is when copying files from the RAID array to RAMdisk or a null device. The rest of the time, application processing between accesses is the primary bottleneck.