This is pretty interesting... I think I will stick to the GTX580, the 680 cards seems to be driver crippled as people thought.
3 Images same settings as you used.
Device 1 performance: 166.105 million samples/sec (CPU)
Device 2 performance: 293.831 million samples/sec (GeForce GTX 580)
Total performance: 459.936 million samples/sec
I have
i7-3930K over-clocked to 4.5GHz
64GM RAM
MSI N580GTX Twin Frozr II/OC
win7 64bit
Such a shame of Nvidia to do this.
Then moving up to Ultra on these 3 particular image I got about: 333 million samples/sec (GeForce GTX 580)
Hey WG
I am running
i7-3820 @3.6GHz ..overclocked to 4.0GHz
32GB RAM
win7 64 version of 0.9.0 pre release
3 images and set to High Arbitrary Smooth (100,000 polys)
Device 1 performance: 118.964 million samples/sec (CPU)
Device 2 performance: 199.001 million samples/sec (GeForce GTX 680)
Total performance: 317.964 million samples/sec