Hi Heini
I would've thought you'd get at least a 10 FPS gain going to the 1080. I just checked your specs and that is an awesome monitor you have, an IPS display at 144hz.
How many frames per second do you get in hitman, do you max out the quality settings and use antialiasing?
You can get better image quality by raising the resolution using the DSR factors setting in the NVidia control panel. What this does is allows you to go 4x the native resolution of your monitor, far beyond 4k, you'll be able to go higher than 2560 x 1440 in the game settings.
All you do is open the Nvidia control panel, Manage 3d settings, DSR - Factors. Click the little arrow, check all the boxes click ok and click apply.
Now you can go higher than 2560 x 1440 resolution in games, you can bump the resolution up one step at a time until performance becomes slow and at a point it will, you won't come close to maxing out the resolution.
The higher the resolution the more crisp, clear and detailed the image, you can play at 4k which is 3840 x 2160.