https://www.youtube.com/watch?v=9f5JQrnOwTE
Another review this time of the 8600k and 8400. 8600k beats the 7700k overall, and the 8400 is very competitive against the 7700k. Interesting to note he did a 2500k at the end of the review and it gets annihilated. So i think it's safe to say that the 2500k is past it's used by date for gamers (chuck in a 3770k though for a nice boost)
As a (soon to be ex) 2500K owner, I would have to agree. Its bottlenecking my R9 Fury which is just a mid range GPU by todays standards. Going to drop in a 2600K for a cheap upgrade, should tide me over until the supply issues with CFL are solved and DDR4 prices come down a bit (hopefully).