Not even remotely true. There have been many GPU generations where the performance increase was 70-100%. 9700Pro vs. 8500, X850XT PE vs. 9800XT, 6800UE vs. 5950U in all DX9 games, 8800GTX vs. 7900GTX, In this case 880 will not outperform 780Ti SLi since it's slated to be a GM204 chip.
I wouldn't buy a 2nd 780Ti unless you need more performance. Otherwise you are just wasting money. Put it aside for the next generation upgrade. When next gen cards come out, you can always resell the 780Ti and use the savings from not buying a 2nd 780Ti to get something much faster.
If you are going to get a 4K monitor soon, sell the 780Ti and get dual R9 290s. That would provide the best bang-for-the-buck upgrade without needing to wait for 880. There have already been sales on R9 290s after-market for $370.