Even though I regularly maintain each system unit by blowing out dust, etc, they do run 24x7x365 so I couldn't in all good conscience sell any motherboards or GPUs in the aftermarket. I've given away a bunch of stuff and have a whole lot more to give away as soon as I can package it up for Fedex (AMD Fury X's, Nvidia 980's, 980ti's, looking around it's about a dozen GPUs). After about 2 to 3 years, I upgrade each system unit with a new motherboard and GPUs, and every 5 years drop in a new power supply. It keeps my sanity in check by not chasing down intermittent hardware failures when equipment gets too old.
In terms of the comparison between the Asus Z170-WS and the ASRock X99 OC Formula, the Asus only has enough room between the PCIe slots to have two GPUs the way I configure my systems. There needs to be enough room between each GPU to allow for adequate cooling, which means having three full slots available per GPU. Putting one GPU right next to another will create an enormous amount of heat and will not only negatively impact performance, but shorten the life of the GPUs. Looking at the picture of the Z170-WS, PCIe slot 1 could host an x16 PCIe GPU, but to get three full slots between slot 1 and the next available slot, which would be slot 4, it turns out that slot 4 is physically an x8 slot so the x16 GPU won't fit even though it is compatible electrically. The next slot the second x16 GPU would fit in is slot 5, and even though a 3rd GPU would fit in the last slot, the problem is it would be right up against the second GPU. This MB wouldn't work for me because I like using 3 GPUs per system and due to the lack of properly spaced slots for adequate cooling, this particular motherboard won't do it. In addition, there looks like an internal USB 2.0 header and what appear to be internal power and reset buttons that would almost definitely interfere with properly seating a GPU in the last slot. I'm sure this motherboard also has a PLX chip to get the 4 x8 PCIe lanes to operate, since the 1151 socketed CPU only natively supports 16 lanes. That may not be an issue, but anytime a switch is involved, some level of performance is lost and another component adds yet one more reliability issue.
On the ASRock x99 OC, there is enough space to get 3 GPUs mounted using 3 full slot widths for cooling purposes See the Newegg pic here:
https://www.newegg.com/Product/Product. ... 6813157596 . However, the case you use has to have room for 8 slots. Assuming it does, then 3 GPUs can be mounted with enough cooling space. You'll note that there is also a USB 2.0 header, but it's located near the back of the last slot, so it only slightly interferes with mounting the 3rd GPU, but it still "clicks" into place. It also has an extra molex connector that's turned 90 degrees, so an additional power connector can be used to augment power to the PCIe lanes and it does not interfere with mounting the GPU just above it. There also isn't a PLX chip on the board which might improve reliability and latency. It's rather surprising how few motherboards allow for this configuration.
In terms of "only" a 10% loss of performance by going to an x4 interface, that becomes a pretty big PPD penalty when you're using Titan X pascal cards. Right now, a Titan X pacal is processing an 11402 work unit with a frame time of 1:33 and a PPD estimate of 1,215,842. A 10% hit is 121,584 PPD. Over a month of folding that's about 3.6 million points, over a year 44ish million points given up by using an x4 slot vs an x8 slot.
FYI, a rack mountable case like the Chenbro RM-41300
https://www.newegg.com/Product/Product. ... 6811123178 has 8 slots and comes with 3x 120mm fans. I modify each case by adding another front 120mm fan and cutting a 120mm hole on each side of the case and mount two more 120mm fans blowing into the case, and add an 80mm fan in the front (comes with two 80mm fans in the rear). I also cut an 80mm hole on the left side located up and to the rear of the case and rivet an 80mm fan grill in place. The airflow is then 6 x 120mm fans and 1 x 80mm fan blowing into the case and two 80mm fans blowing out. Needless to say there is an enormous amount of airflow that exits out between the GPUs and through the 80mm hole at the rear. All the GPUs run about 58 to 70 deg C with a small overclock (temp variation depends on core and work unit type).
I'm not endorsing any vendor over another, I'm simply referencing what has worked for me. I'm sure there are other configurations that work just as well using components from other manufacturers. Hope this helps!
If they can't utilize what I'm sending them, I'll shoot it your way. Just PM me your physical mailing address.