1080p seems to be a hardcoded max in the game as well. I tried editing the registry settings to use 2560x1440p which works during the intro logos but as soon as the main menu appears, it gets forced back down to 1080p. (HKEY_CURRENT_USER\Software\Buckshot Software\Project Warlock if you're curious)
I get the aesthetic style of pixel art but higher resolutions mean that those pixels are much sharper. Running full screen 1080p on a 1440p monitor makes everything slightly blurry and running windowed is annoyingly small. I always prefer to run at max resolution, even if I have to lower other settings as 1:1 resolution scale produces the sharpest image.
Please put a patch in to allow arbitrarily high resolutions (or even up to just 4k).