GTX 680 was the top end card for the 600 series but bear in mind that the GTX 670/680 were GK104 GPUs. That designation 104 used to mean midrange GPUs. The GK110, the real high end Keplers didn't come out for almost a year later. If the GTX 880 is a GM204 as rumored and still on the 28nm process then it's a difficult call to know whether it will beat the GTX 780Ti. If it were on the 20nm process then I would think that it could easily beat a 780Ti because Nvidia could push the clocks up for the same wattage used and the Maxwell design is more efficient anyway.