FunSurfer :
To get the best answer to your question you will need to enter the gaming Russian site "GameGpu" with the search line "GameGpu [name of the game] test". You will need to enter games from the last couple of months to see the new benchmarks. In the site you will need to scroll down to the CPU benchmark, you will see there the i5-2500k and the i3-2100, change the gpu in the drop list from the default GTX1080 Ti to GTX 1060 or GTX 1070, your CPU should be 10% less then the 2500k and always better then the 2100.
Example:
http://gamegpu.com/action-/-fps-/-tps/hellblade-senua-s-sacrifice-test-gpu-cpu
Sites like this a generally VERY POOR at describing the performance you can expect. As is thebottlenecker.com and sites like it, they literally get mocked by the mods, regularly. If you really want to know for certain what your CPU is capable of do this instead.
1. Run a frame counter like fraps
2. Choose a title you play
3. Lower all resolution and image settings to as low as they go
4. Look at what your frame rate is.
The answer you get form this is the frame rate your CPU will always at least be capable of at any resolution and graphic settings (not including CPU based physics) assuming your GPU is powerful enough. CPUs are good for far longer then folks realize in gaming. So test your CPU out and really see what it is capble of. Regardless if you plan on an upgrade soon, i wouldn't worry about a short term "maybe" bottleneck. Just enjoy your GTX 1070.