Do your research before claiming anything.
Claiming that the 6990m is even similar in performance to the 680m is laughable and tells me that you only did a quick Google search and accepted the first forum post as correct. This is not a good practice.
The 6990m was similar in performance to the 580m, not the 680m. There is a large gap between the two. Seeing as how the 580m SLI is still less powerful, your claim leaves me confused.
Also, by your own link, the 680m is...