So I have a 2500k @ 5Ghz and a 1070. More often than not the 2500k bottlenecks the 1070 but it was probably still worth the investment.
Previously I had a 670 and the framerate increase from the upgrade in most games was significant. However there are a few badly optimised games which really want more than my 2500k has to offer. Arma and Pubattlegrounds being good examples. In Battlegrounds my CPU is sitting at or close to 100% load at all times causing a low average framerate and horrible, constant stuttering. Meanwhile my 1070 will often be idling around 40% load. It is a bad experience.
weird, my 2500k doesnt even get close to 100% in battlegrounds.