Your info is bad...
The HD6990 competes with the GTX 680: and overclocked GTX 680 can defeat a 6990. Although they perform the same, the 6990 has 2x the power consumption and makes much more noise......
The GTX 690 gives around 190% the performance of the GTX 680 and is 2x the performacne of the HD 6990
But the links which I have provided don't say so..
Memory Bandwidth
The Radeon HD 6990 should in
theory be quite a bit faster than
the Geforce GTX 680 in general.
(explain)
Radeon HD 6990
320000 MB/sec
Geforce GTX 680
192256 MB/sec
Difference: 127744 (66%)
Texel Rate
The Radeon HD 6990 is quite a
bit (about 24%) more effective at
texture filtering than the Geforce
GTX 680. (explain)
Radeon HD 6990
159360 Mtexels/sec
Geforce GTX 680
128768 Mtexels/sec
Difference: 30592 (24%)
Pixel Rate
The Radeon HD 6990 is quite a
bit (more or less 65%) faster with
regards to full screen anti-
aliasing than the Geforce GTX
680, and also able to handle
higher screen resolutions more
effectively. (explain)
Radeon HD 6990
53120 Mpixels/sec
Geforce GTX 680
32192 Mpixels/sec
Difference: 20928 (65%)
*tpucdn.com/reviews/NVIDIA/GeForce_GTX_680/images/perfrel.gif
that shows the the radeon 6990 is 2% faster.. .. than a stock gtx 680.. a little overclock and the gtx 680 will overtake the radeon....
those specs influence performance but then kepler architecture is amazing....