In this thread I want to do two things:

1.) Share which adjustments were necessary to get 8 DDR4 modules of a total memory amount of 64 GB running @2400GHz, which was not out-of the box in my case. The adjustments I needed may help others as a starting point.
2.) Ask for advice which screws I might try for a higher memory clock.

First of all, I am running a 5820K@4.4GHz in adaptive mode at 1.32 V turbo voltage, rock stable, BIOS V.1701. I purchased new RAM: HyperX FURY DDR4-2666 15-17-17-35, 1.2V (double-sided with Hynix chips). As 64 GB Kits seemed either overpriced or have currently limited availibilty, I decided to do some gambling and ordered two kits of 4 modules each, at reasonable costs of total 410 Euro (what you are not advised to do at all unless you are a little masochistic https://rog.asus.com/forum/showthread.php?57038-Don%92t-combine-memory-kits!-The-meat-and-potatoes-o...).

1.) After installing all eight modules, I was not surprised that XMP 2666MHz was not stable. But I was surprised that also 2133 MHz on “auto” was not stable, even not at timing cycles of the 2666MHz profile (this is for example 15-17-17-35).
But I did achieve stability at 2400 MHz for the following intervals of voltage adjustments (I havent tested for 2133 MHz):

DRAM Voltage 1.285-1.325 V
VTTDDR Voltage 0.67-0.7 V
VPP DDR Voltage 2.62-2.87V
DRAM frequency 2400 MHz (where DRAM timings can be tightened a little).

Amazingly for all three voltages only certain intervals are possible, exceeding them has adverse effects!
Note that some characteristics I found may be rather determined by the CPU’s memory controller (IMC) than the memory itself.


2.) I understand for highest overclocks only 4 modules are advised but I wanted to see all the slots populated 😛
Now, I am flirting with getting maximum speed out of the RAM and maybe even reaching 2666 MHz. I tried many adjustments at 2600 MHz but without achieving stability: SuperPI for memory stability testing gives rounding errors (best after 10 minutes).
What can one specifically advise for overclocking 8 modules? Which screws may I try now?
It is great the Rampage V offers so many memory adjustment options unfortunately little information about them can be found, as to DDR4 overclocking.
So what is worth trying with these settings:

PLL Termination Voltage (I have left untouched)
CPU Input Voltage (I tried raising by 0.05-0.15 V with no success)
CPU System Agent (Auto will set it on 0.96V and that seems a good guess - manually I found stability at 2400 MHz between 0.92V and 1.08V)
DRAM Swizzling (setting bit 0 and 2 to disables makes my system awfully unstable)
DQ Sense?
DQS Sense?
CMD Sense?
CTL Sense?
DRAM CLk Period?
BIOS the DRAM Timing Control/IO Control menu ??
DRAM Switching frequency?