For my new build I will be using:
Core i7-6850k
2x EVGA GTX 1080 FTW
an NVMe drive - preferred drive is 1.2TB Intel 750, could use Samsung 950

Since the CPU supports 40 lanes I want to have the GPU's at x16 and the drive at x4. I am currently looking at three motherboards and was hoping someone can tell me if the will work or not. After reading the ROG forums I am concerned that I could wind up slowing down the primary graphics slot. I know there is little difference between x16 and x8 but if I am spending this much money I want everything right.

Asus Rampage V Edition 10
The GPUs would go in slots 1 and 3. That leaves slot 4, M.2 and U.2 available.
However I saw on the Asus ROG boards that using slot 4 causes issues and that if you use the U.2 slot it turns on the M.2 and reduces slot 1 to x8.
So it looks like only an M2. drive would work with this board?

The GPUs would go in slots 1 and 5. That leaves slot 6 and M.2 available.
So the Intel 750 would have to go in slot 6 or a Samsung in M.2.
Does anyone have any experience doing this?

MSI X99AGodlike Gaming Carbon
GPUs would go in Slot 1 and 4. That leaves slot 5 and M.2 available
The Intel 750 could go in slot 5 or use the U.2 adapter in the M.2 slot or Samsung 950 in M.2 slot
Does anyone have any experience doing this?