Seen this.

The challenges with the scrypt hashing are a bit greater than just using the funnel shifter for rotation. One issue is the speed and efficiency of memory access, the other issue is getting enough occupancy on Kepler's SMX (multiprocessor) units - shared memory and register limits are an issue. This mainly affects the GTX 660Ti, GTX 670, 680 and Titan devices which currently perform rather poor in comparison to the 5xx series.