Yep 2500k is bottleneck in new games.Even 2500k at 4.8ghz its slower than skylake at 3ghzPlenty of recommendations for the 1070 coming in but I could see so much of its potential wasted by a cpu bottleneck here. Modern games are really starting to show the limitations of i5s, especially given the IPC disadvantage of Sandy vs Skylake. I'd take an upgrade to a 2600k and a 1060/480 over just slapping in a 1070 and hoping the 2500k doesn't choke it.
If you have $400 to spend I'd seriously consider continuing to save for an entirely new system rather than putting it into an older one.
https://forums.anandtech.com/thread...astered-gpu-perf.2491354/page-3#post-38603452
i recommend you first upgrade PC(CPU/MB/RAM) and then look at new GPU.By that time new better cards can be already out(vega/pascal refresh)