I can't help you with the figures you're requesting. I'm using 5870's, I would like to see someone go over the top of this build using 5970's or 6990's. I would like to know their results.
7-8 Single gpus work on one MSI Big Bang with no molex.
12 volt power cables from PSU are not hot or warm to the touch.
There is only one issue so far with running eight. Stability in the operating system.
Windows 7 will recognize 8 gpus with a lot of tweaks. ATI CCC can not be installed and I needed to build my rig up slowly after installation.
Linux Coin recognizes all 8 gpu's but from my experience lc has never been a completely stable platform.
Big A Miner Thing does not support 8 gpu's long term for some reason, but will support 7.
From Aaron Wolfe, creator of BAMT, when asked about the problem:
your best bet is to just pull the 8th card.