OK experts, I'm hoping y'all can help me out here.
My next personal build (whenever that is) is going to be a virtualization monster. I'm a software engineer and I frequently deal with complicated multi-VM deployments, so having a system where I can go crazy with Vagrant, Puppet, LXC, or whatever and get it working or failing fast is attractive to me. Also because e-peen.
A 64 GB quad-channel system like the ASUS X79 Deluxe (TR review http://techreport.com/review/25310/asus-x79-deluxe-motherboard-reviewed, ASUS https://www.asus.com/Motherboards/X79DELUXE/) or the "workstation" variant (https://www.asus.com/Motherboards/P9X79E_WS/) would be a candidate if I were buying today. Max non-overclocked RAM speed is 1866.
Mighty indeed would be 256 GB (registered RAM) in their Xeon Dual-Proc: https://www.asus.com/Motherboards/Z9PED8_WS/. Max RAM speed is also 1866 (and nobody overclocks in that space).
What I'm wondering is, in either case, whether that 1866 is still achievable with every memory slot filled with the max supported RAM. I can't find the information even from the information available in their QVLs and mobo manuals.
I remember feeling burned a few years ago when I tricked out my new system with expensive high-end RAM filling both channels (this was a big deal at the time), but it turned the command rate down from 1T to 2T -- and my Prime95 scores tanked. ISTR it also being harder to make stable without lowering the speed. I basically shouldn't have spent the extra money on the RAM -- lower-spec RAM would have been fine.
So -- what's the story? Is it worth buying high-dollar RAM (from the QVL) in hopes of hitting those rated speeds? If not, at what point does the "hit" start to take hold? Is it more about number of slots filled, number of channels used, total RAM size, ...?