I hate to do it, but even my i5-2500k is holding back my SLI setup in 3D Mark Vantage. I have to run my ram in single channel because my board is broken, but thats only going to cost me a few points probably since my read/write speeds are only around 14k that should place it on par with a AMD setup in dual channel mode.
Despite that and from personal experience with AMD chips at 4.3-4.5GHz I will be so bold as to say AMD is so far behind that I have higher GPU usage with SLI than any of you will have with a single high powered video card... Yes, AMD is a bottleneck. Do you care? I dunno, but the op never asked if you cared, he asked if it was going to hold back a 6990 and if any of you think it won't you're smoking some good stuff.
AMD simply can not compete in the high end market, and with the release of a $220 Sandy Bridge chip they can't compete anywhere any longer but the ultra low end segment.