My mistake, I was tired and typed mATX = Micro ATX, but you're right and I was wrong. mATX = uATX.
The 28 PCIe 3.0 lanes supported by an i7-5820K are still wasted if you only have a single PCIe 3.0 slot to plug into. Motherboards and GPUs which conform to the x32 PCIe 3.0 standard don't exist yet (and it seems they never will, although it's not impossible), so uATX is still locked to x16 PCIe 3.0 and no more. I don't personally see any merit in an X99 uATX system when a Z97 uATX will be better and cost less.
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[/Korth]