If we dive deeper into various motherboards, we can see this isn't the case, yet what you said keeps being repeated on forums like gospel.
1) Scenario with a lower end X99 motherboard - For example Asrock X99 Extreme 4:
5820K has 28 lanes. That means on most motherboard, it can work as 16x/8x + M.2 PCIe 3.0 x4 configuration.
16x for the 1st GPU
8x for the 2nd GPU
4x for the PCIe 3.0 x4 M.2 SSD
"*
If you install CPU with 28 lanes, PCIE1/PCIE3/PCIE5 will run at x16/x8/
x4.
What happens when we insert the M.2 PCIe SSD? The 3rd PCIe slot gets disabled - but this is for the 3rd videocard so it's irrelevant.
**If M.2 PCI Express module is installed, PCIE5 slot will be disabled.
***If you install CPU with 28 lanes, 3-Way SLI™ is not supported.
****To support 3-Way SLI™, please install the CPU with 40 lanes."
http://www.asrock.com/mb/Intel/X99 Extreme4/?cat=Specifications
Going from PCIe 3.0 x16 to x8 is a 0-1% performance hit.
In this case, on a lower end X99 board, 5930K is only necessary when running
3 GPUs or 3GPUs + PCIe SSD or if running 2 GPUs +
2x PCIe SSDs, etc.
2) Scenario - higher end X99 motherboard such as Asrock X99 Professional:
It gets better, some higher end boards can do Tri-SLI/Tri-Fire + M.2 PCIe SSD with a 5820K.
"* If you install CPU with 28 lanes, PCIE1/PCIE2/
PCIE3/PCIE4/PCIE5 will run at x16/x0/x4/x8/x0 or x8/x8/x4/x8/
x0, and
PCIE5 will be disabled.
** To support 3-Way CrossFireX™ and 3-Way SLI™ when using CPU with 28 lanes, please install VGA cards to
PCIE1/PCIE2/PCIE4 (x8/x8/x8). * If Ultra M.2 PCI Express module is installed, PCIE3 slot will be disabled."
That means it's possible to run 5820K + GTX980 Tri-SLI in PCIe slots 1, 2, 4 and still have PCIe 3.0 x4 SSD.
http://www.asrock.com/mb/Intel/Fatal1ty X99 Professional3.1/
In conclusion, if someone is buying a 5930K over 5820K for dual GPUs and a single M.2. PCIe 3.0 x4 SSD, they are just wasting $ that could be used towards better cooling, monitor, games, etc. Since some boards can even support 8x/8x/8x + x4 on a 5820K, it requires additional research to be able to discount the 5820K as well.