The core i7 2500K & 2600K are really great cpu's!
But if you want the best chess performance then is the 980x much faster!
Don't forget,the 980x has still 2cores more.
Of course 980X has two cores more, but maybe it is not enough (but to be sure we should have a KN/s comparative test with these processors) to be able to compensate the large difference of about ~1600 MHz (6X~4200 vs. 4X~5800 +advanced/newer/better CPU/architecture) compared to 2600 i7.
Note, do not forget that most of the cores can not achieve (for most of the chess engines) more than 80-86% of efficiency compared to the primary core.
In the case of the Rybka engine it might look like this:
i7 980X ~4200 MHz + ~3276(~78% eff. per core)x5 Cores  = 20580 Total effic. score/speed
i7 2600 ~5800 Mhz + ~4930(~85% eff. per core)X3 Cores  = 20590 Total effic. score/speed
>980X can also run with 5-6 GHz !
But not (100% stable) on Air. :)
>Demchuk it shows every day on Chessbase
Actually :), this is not any type of proof, because his opponents they not possesses i7 2600@5800MHz :) and some of them have a weaker opening books than he. ;)