Cayman XT [Radeon 6970] outputs 2.7 TFLOPS in Single-Precision and 675 GFLOPS in Dual Precision. [1] I think the 6990 is just two of those, or at least that's normally their convention.

[1] http://www.brightsideofnews.com/news/2010/12/13/amd-radeon-h...