The i7 2600K can wipe thew floor with all but the 990X and at the price of it I wouldn't bother unless you needed to run some heavy multi threading tasks.
Problem is, it wipes the floor with the 990X, too. The only situation where the hexa-cores are better ( sometimes by a big margin ) is heavy load on all cores during rendering, transcoding, etc.
Even if you wanted to run raw processing power tasks I'd think of going with 2 SB rigs for a similar price to one 990X rig.
There's also the issue of SB reaching higher overclocks than the 990X. Some of the 2500K/2600K chips go over 5 Ghz mark ( on air, too ) while the 990X will struggle to go much higher than 4.6-4.8 Ghz.