Problem with GF110 is that it is too power hungry... Look at power draw of 580SLI - 719W
http://www.guru3d.com/article/geforce-gtx-580-sli-review/14
This is what 590 would draw if designed to be all it can be.
Here is another bench of many cards
http://www.anandtech.com/show/4209/amds-radeon-hd-6990-the-new-single-card-king/18
580SLI is claimed to pull 620W... don't know why the 100W difference
But 6970CF pulls 564W, which is the worst case scenario for a 6990 power draw. The problem with going that high in wattage is that you can not cool it with air AND you are really really breaking pcie spec. PCIE2.0 tops at 450w? The 8-pin connectors are 150W each and I think the slot is 150W also? Slot used to be 75 in pcie 1.0... So, we have 6990 which at 830MHz conforms to PCIe 2.0 and breaks it at 880MHz. And then we have GTX590 which breaks it at stock... The reason being is b/c 590 is factory overclocked to actually be competitive with 6990. This is why there is very little if any at all OC margin on most 590s.
So, what we have is:
6990 at 830MHz conforms to PCIe2.0
6990 at 880MHz breaks PCIE2.0
590GTX at stock breaks PCIE2.0
590GTX OC stays under PCIE2.0 after it explodes.