That depends fully on your usage, and how many cores the programs you use benefit from.
Obviously the vast majority of programs I use don't benefit from more than four cores, so I bought a $200 chip that outperforms your $600 chip in nearly every task I use my computer for.
However since your usage is different than mine, your cpu probably out performs mine in the vast majority of the programs you use.
Hence comparing an i5-2500k to a eight core cpu is folly in the first place, since they're meant for different tasks it would seem. The eight core FX for $220 seems to offer compelling price vs performance, if the user is someone like yourself. However if that user is someone like myself, it's not even worth $110.
So should we compare the i5-2500k to any FX cpu besides the quad core? I don't know, I guess that remains a question for the end user. Eight cores vs four is neither irrelevant, nor is it relevant, in absolution, that decision remains at the sole discretion of the end user.
Makes the most sense to me.