This is true but at present, we have a 780Ti that has not had its full potential realised yet also. I did some math (that I am seriously bad at) for comparison as we stand now. Both new and both on immature drivers.
Going from the stock Vs stock results on the
Techpower up site and using the full HD results, it stands as this:
Assassins Creed 3 = 780Ti wins by 17%
BF3 = 780Ti wins by 11%
BioShock Infinite = 780Ti wins by 18%
Black Ops 2 = 780Ti wins by 23%
Call of Juarez = 780Ti wins by 5%
Crysis 3 = 780Ti wins by 11%
Diablo 3 = 780Ti wins by 21%
FarCry 3 = 780Ti wins by 14%
Grid 2 = 780Ti wins by 7%
MetroLL = 780Ti wins by 10%
Splinter cell: Blacklist = 780Ti wins by 37%
Sleeping dogs = 290X wins by 10%
Hitman Absolution = 290X wins by 1%
You can clearly see the 780Ti is winning and by quite a margin. The 780Ti is faster in 11 of the 13 games tested and when added together, they result in 12.5% faster.
Now when you add an overclocked 780Ti into the equasion, does 18% seem doable over the stock 290X? I would say easily. Of course there will be overclocked editions of the 290X as well, so maybe an unfair bench comparison however, the 780Ti IS 12.5% faster overall in the majority of games and so many of those games benched are AMD Gaming Evolved titles. Does it justify the extra £100 price premium? 3 free games and that original £100 comes down a fair chunk cheaper still.