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I have compared an EVGA GTX 680 4gb Classified, overclocked with a slightly overclocked EVGA GTX Titan Superclocked. I used the test file referred to in this thread and ran the latest RT. Here are the results, at 1920x1152 sized image:
680 OpenCL = 1m 48.3s
680 CUDA = 0m 44.1s
Titan OpenCL = 1m 42.7s
Titan CUDA = 0m 29.9s
I am not an expert VRay user, by any means. I have appreciated all the help I have received in this forum, so I wanted to share my interesting results!
Also - the 680 GTX was tweaked for max performance, the Titan is not.