About to make some upgrades to my build, need recommendations

I know for sure I'm upgrading from Air Cooling. Planning on going to the H100i v2

Now after that I'm planning on getting 8gb of ram (2x 4gb sticks) to make for a total of 16 gb of ram in my system.

From there is where I need advice

I'm either going to upgrade my gpu or my cpu

I'm currently running a EVGA SC 1070 and a i5 6600k (Overclocked)

Now which would I get the best performance increase at 1440p, cpu or gpu?

If I'm upgrading my gpu should I get a second 1070 and sli or should I just get a 1080?

You can find my full build in my flair. Thanks for any help