the only thing i would say about this is with a titan you would only have to power 1 gpu, and its a powerful one are you ever going to notice a difference in real world gaming? (besides generating less heat in your case + using less power(?))
I really don`t know because i never used the 680 SLI before, and i have the issue that i play at 2560x1440, so i really don`t know if it will make some difference