First of all the OC speed is 6% higher than default one,no brainer to deduce how much faster 6990 vanilla is. Second,in the NDA document(so not the official one from AMD yet) the compared card from NV is its best currently,from uarchitecture perspective(sure OC can give you ~5% more).Don't blame AMD for NV's lack of better product. If there was a dual gpu monster made of 2 Fermis AMD would have to include it in the chart. This way they made best Vs best comparison and it's perfectly valid.
And if you expect that NV can put two full fledged Fermi GPUs from GTX580 on one board with the same spec per GPU then I think you are overly optimistic.
Bookmarks