There have been no significant speed bumps with CPUs since Sandy bridge and anything from a 2500k to 6700k will have little impact on gaming. That being said, the advances in GPus have been rather impressive .... so much so that we now appear to be seeing SLI scaling as being limited by CPU Power.
At 4k, the impact of adding a 2nd card remains a worthy consideration.... but at 1080p and 1440p, adding that 2nd card has a lesser impact with the 10xx series, something we haven't seen before and it seems likely that CPU limitations may be at least partially to blame.
Something is still limiting SLI scaling....and something is limiting single card performance. As we can see below, a faster GFX card brings more to the table at higher resolutions where GFX is the primary limiter of performance
The 1080 is 22% faster than the 1070 at 4k
The 1080 is 21% faster than the 1070 at 1440p
The 1080 is 18% faster than the 1070 at 1080p
That's with a 6700k. With a lesser processor, I would expect that spread to be a bit larger but by no means a "holy crap" moment. I mean if you are getting 80+ fps in Witcher 3 or 130 fps in BF4 @ 1080p, are you really gong to be all that upset if it drops to 75 / 120 ? The CPU / GPU combination will still provide more than satisfactory performance at any resolution