Is your case limiting you? The only way to tell for sure is to remove the side panel, then put a box fan in front of it. If your temps go down substantially, then yes. Things can be done with mid-tower cases such as changing the fan configuration or buying more efficient fans. I don't think that's the case here.
Any GTX card 1070 or above is going to run at around 73c and up, and temps like 69c to 73c are the custom cards like the MSI Gaming X, EVGA FTW, and Asus Strix triple fan. Some of the cards with cheaper cooling solutions will run around 80c, all the way up to 84c. The Founder's Edition and other cards with the blower/turbine cooler will run at nearly 84c unless you crank the fan speed up. You can download and use MSI Afterburner to adjust or increase your fan speeds on your GTX 1080 card. I have an EVGA GTX 1080ti FE card and increasing the fan speed does wonders for the temps. The Asus Turbo card uses a blower type cooler, so running at over 80c is normal no matter what case you're using. That's the first thing I recommend doing before you buy new case fans or buy a new case all-together.
My example if from what I've dealt with with my GTX 1080ti FE card. I have my GTX 1080ti in a supertower, which is the largest case you can possibly buy. Even with this Corsair 900d case, if I don't set a fan profile with Afterburner, then my GPU temps will be toasty hot at 83c. With the fans set to 80% the card stays within reasonable temps and the boost clock will go as high as 1910mhz.
Other factors such as ambient temperatures will greatly influence your GPU temps. During the summer my PC with an R9 390x will run as high as 85c, even with the household temps at around 24c. This time of year my household temps are around 20c and the R9 390x won't break much past 70c.
Your NZXT case is very limited with it's fan configuration. Two intakes at the front, a rear fan and a top fan that should be used as an exhaust. realistically, the only thing you can do is find two 140mm front fans that can push more CFM. If the air still stagnates at the top, which it might, then you would have to buy more powerful 140mm exhaust fans as well. It's not really made for watercooling. Although you could add a 280mm radiator or AIO cooler to it, and it look like it could possibly mount a reservoir. You will find trying to add a loop to your NZXT case to be very tedious.
The GTX 1080 card will handle Ultra at 1440p at 60fps for the most part but don't expect to get 165fps at Ultra. The 1080 is just not capable of that. The GTX 1080ti can't even handle that. Are you configuring the setting and turning them to high in order to get high FPS?