Okay, I am lost~
Why would you want to downgrade from an SLI 580 set-up to a 750 ti SLI? You'd lose a lot of performance power, but save money on power lol.
A single 780 non ti is about equal to your 580's in SLI. Though the 780 edges a bit by winning some key categories.
Gtx780 will have a higher core speed, Higher GPU memory, faster memory speed, and faster memory bandwidth. But honestly, why go with this when a cheaper more powerful alternative was just released in the 970?
Also, side note. My argument is with rhetoric, as the 750 and 750ti aren't sli compatible