In order to put the 2014 i7 devils canyon cpu in your board, it's bios needs to be flashed, because your board is 2013 and the cpu is 2014 i think, it wouldnt work without the bios update;
Yea the i7 4790k should give you more fps because of its higher IPC and HT where usable, but only if you drop the quality to medium or medium low, depending on the game, if you go high or very high or even ultra, the cpu won't matter that much because all the stress will be put on the GPU; - and this is 1080p i'm talking about;
If you go 1440p then the GPU will work harder and will need a stronger CPU to follow suit;
Here's a dirty test if you curious about the FPS you gonna get;
Run a game now in 1080p and watch the fps, then divide it by 2 and thats the amount you will get if you go 1440p high settings;
You didnt say what speed your cpu is running at and whether if its overclocked or not;
If you run a stock 4670k then forget high refresh rate gaming esp. on 1440p, it's turbo is on 3.8Ghz and that should give you around 80-90 fps on very high with a gtx 1080, or less if you go crazy with anti aliasing options in the game