1. Upgrading from a i5 2500k to an i7 6700k is extremely difficult to justify. We have already shown that going from Sandy Bridge to Sklyake results in typically a 1% increase in frame rates and sometimes even a decrease in minimum frame rates.
2. You have been misinformed if someone has told you that you will have a significant improvement in gaming from going to an i7. Id expect, depending on the game, anything from -5% to + 10%. Building a new box today ... a i5 / GTX 970 build will crush a i7 / 960 build.
Here the difference between the current i5 and i7
Total War: Attila on ASUS GTX 980 Strix 4GB ($560) = 35.7 vs 35 (i7
loses you 2%)
1080p Ultra, Average Frames Per Second
Grand Theft Auto V on ASUS GTX 980 Strix 4GB ($560) = 73.41 vs 74.09 (i7 gets you 1%)
1080p Very High, Average Frames Per Second
GRID: Autosport on ASUS GTX 980 Strix 4GB ($560) = 172.34 vs 169.88 (i7
loses you 1.5%)
1080p Ultra, Average Frames Per Second
GRID: Autosport on ASUS GTX 980 Strix 4GB ($560) [Minimum FPS] = 146.69 vs 150.78 (i7 gets you 2.8%)
1080p Ultra, Minimum Frames Per Second
Shadow of Mordor on ASUS GTX 980 Strix 4GB ($560) = 99.54 vs 99.60 (i7 gets you 0.0%)
1080p Ultra, Average Frames Per Second
Shadow of Mordor on ASUS GTX 980 Strix 4GB ($560) [Minimum FPS] = 48.26 vs 44.85 (i7
loses you 7.1%)
1080p Ultra, Minimum Frames Per Second
Shadow of Mordor on ASUS GTX 980 Strix 4GB ($560) = 39.35 vs 39.36 (i7 gets you 0.0%)
4K Ultra Settings, Average Frames Per Second
Shadow of Mordor on ASUS GTX 980 Strix 4GB ($560) [Minimum FPS] = 30.00 vs 32.03 (i7 gets you 6.8%)
4K Ultra Settings, Minimum Frames Per Second
As you can see above, the 4.0 Ghz 6700k is losing as much as it beats the 3.5 GHz 6600k
despite the 0.5 Ghz speed advantage.
3. Sandy Bridge was a much better overclocker than Ivy Bridge ... so much so the EK introduced "Naked Ivy" as delidding became prevalent to solve IBs poor heat transference issues. We'd get typical 4.7 - 4.9 Ghz OCs on 2500k / 2600k / 2700/ builds (75C max core / 1.35 volts) ..... 5.0 + was doable ..... Ivy Bridge typically was 4.4 - 4.6.
https://www.ekwb.com/shop/ek-supremacy-precisemount-add-on-naked-ivy
http://www.overclock.net/t/1376206/how-to-delid-your-ivy-bridge-cpu-with-out-a-razor-blade
4. i7 runs about 7C hotter than i5. On some 3770k builds we did, we created multiple BIOS profiles w/ all 8 cores used for day time applications (CAD / Photo / Video Editing) and a separate BIOS profile for gaming (no hyperthreading = 4 cores) which was able to maintain higher OCs, run cooler and get higher fps.
5. When building a new box, if not overclocking, you get a nice 0.5 Ghz boost with today's CPUs going form 6600k to 6700k... and advantage that we saw above brought us as many fps decreases and increases. The 3770k comes with just a 0.1 Ghz advantage over the 3570k
Multiplayer games do see more of an increase increase than single player and you may see as much as a 10 fps (15%) increase in some games. Of course if you handpick your titles, you can produce an argument either way. However, for the same investment in a new CPU, you'd be much, much better off, going to SLI / CF for an improvement that dwarfs that CPU gain (-7 to 15%) with an average increase of 70%.