extreme 4 / z97x killer and usb 3.1/c

Yes you are right about both situations.

All the socket 1150 processors you could use with those boards have 16 PCIe 3.0 lanes. That is a limitation we can blame on Intel.

So strictly speaking it's not really the use of more than one PCIe 3.0 x16 slot that limits a single video card to x8 (although the result is the same), but the amount of PCIe 3.0 lanes that are available.

Regardless, your descriptions are just as accurate about the final results.

Most gamers say that running a single video card at PCIe 3.0 x8 does not reduce performance much if at all. The SLI situation is not the same.

At present, there are no GPUs that can fully saturate PCIE 3.0 x8, at least that is what benchmarks are showing. I think the closest to doing this is the Titan Z. PCIE 3.0 x8 is more or less = to PCIE 2.0 x16 in bandwidth. 

hellazari wrote: yes, thats what I thought, thanks for clearing that out. but what about the extreme 6 board? he has a pcie 2.0 x16 slot which runs at x2 and from my understanding this slot is not affecting the 2 pcie 3.0 x16 slots speed. can i fit the USB 3.1 card into the pcie 2.0 x16 slot and the GPU will still run at X16 speed?  if yes, pcie 2.0 at x2 speed is 0.8Gbit speed so this slot will bottleneck the card  I guess.

Again you are correct.

The PCIe 2.0 x16 slot (x2 lanes) is provided by the Z97 chipset, not the CPU. So using that slot will not use any of the PCIe 3.0 lanes, and all will be available to the video card(s).

Another thing to know is the exact specs of the USB 3.1 card, which can be seen here:

http://www.asrock.com/mb/spec/card.asp?Model=USB%203.1/A%2bC

Note that this card is physically x4, but is x2 electrically, so uses only two lanes.

I don't know why you said PCIe 2.0 x2 speed is 0.8Gbits, since one PCIe 2.0 lane is supposed to be 4Gbit/s max effective transfer rate, taking into account overhead.

The card will be slightly bottlenecked when used in a PCIe 2.0 lane slot, since that would not provide the 10Gbit/s (or more) speed that two PCIe 3.0 lanes would provide.