If you can find out for a fact that Gigabyte have bothered using a BIOS for it that will take Devil's Canyon from the get go, then go for it. Maybe they have. It would just be the first I hear of a Z87 capable of doing so out of the box (other than some Asus boards which don't require a CPU installed to carry out a BIOS flash). Don't rely on the revision date alone.
The extensions are not very long.
Things have improved a bit on that front over the years. A quality PSU these days (and for a while now) can have a more powerful single rail than two or four rails combined in PSUs of years past. Obviously depending on the models. Just for argument's sake.
That 650W model could run an O/C'd CPU and 2 x 960s for example. Or a non-O/C'd CPU and non-OC'd 2 x 970s.
It's CPU and GPU dependent. If you went for a 290X then it'll only run one, and that's what you'd want for such a card. 550W would be good, 650w even better.
Addressed that here:
There is no need to actually slot the adapter in a PCI slot. Just place it somewhere below the PSU shroud for example, find the brightness level you're happy with, and close up the case once you're done building.
Yes. The overclocking ability of CPUs is dependent on the CPU itself for the vast part. A quality motherboard just helps eke out that little bit extra, and find stability on overclocks that would not be quite as stable on other boards. But if you land a great chip you'll easily have a better O/C than someone with a £200 board and a poor chip.
Anything up to 2400MHz is fine on Z97. Beyond that it starts getting iffy progressively, especially when combined with a CPU overclock. The gains beyond 2133MHz are minimal in general.
No. There's a BCLK strap now* and the memory frequency runs independently of CPU frequency (even though the CPU has a memory controller aboard). So you can up the CPU multiplier without affecting memory frequency for example. This article may be useful as an introduction to how it works: http://www.overclockers.com/3step-guide-to-overclock-intel-haswell/
