I can't see how the 780 could be anywhere near as good as Titan unless it gives more which I don't think is possible.
By all accounts the 780 will be a cut down Titan, cores that didn't make the grade.
Unless that's changed of course.
According to rumored specs (taken from the database at TechPowerUP), the 780 will use 2304 shaders, compared to the 2688 in the Titan. It will be clocked faster, at 863 MHz compared to 836 (900 and 875 boosted)
So overall the 780 will be around 10% slower than the Titan at stock speeds, assuming all resources are used.