So how about GCN (HD7970)? Is this supported/faster/...?

Would it be a possible solution to mix vendors (say 4x dual AMD, 3x dual nVidia)? Or is it out of the question to ever achieve this in any reasonable way (and should I better forget about running dual cards in this way)?

It seems stupid that there are many boards with >4 GPU slots if >4 dual GPUs is impossible. :/

When not using a case, it's not so difficult to use multiple PSUs, and with PCI-e extender cables with molex connector, this seems fairly doable, since the power won't go through the motherboard. Or am I wrong in this?

I find various sources on this forum claiming that the AMD linux drivers only support up to 8 GPUs (i.e. max 4 dual GPUs).
I however find a lot of conflicting information on this forum when it comes to the number of GPUs that can be driven on windows (6, 8, 256, ...).

It's physically possible to build a rig with 7x6990, on a standard motherboard with 7 PCI-e slots (of any kind). But will the hardware (address bits?) and the windows drivers be able to handle this? Or will there be other software problems? I couldn't find an example here of a rig running >8 GPUs on the same motherboard, nor could I find any explaining why or why not it would be (im)possible.