Thanks I didn't know that. I tried editing the file but it seems useless in borderless, resolution is ignored only ResolutionScale seems to matter.
In windowed I am able to set the res to 2560*1440 (even if the settings menu doesn't want to show that) and get way better performance but I get the fat top border and obviously the rest of the screen isn't hidden.
Did you make sure that the game itself has the right volume (settings can vary per application)? You can check by right clicking on the volume icon and select "Open Volume Mixer". In the popup you can see what the volume is for all active applications.
This is the troubleshooting section so i presume you're in the right place ;).
You can try to validate your game file to see if there's a corrupt file. (right click on your game in the library, select properties, then click on Installed File and press the "Verify integrity of game files" button).
If that doesn't work, remove your profile settings from the "My Games\ArmaReforger" folder and try again.
As of DX12 Microsoft has replaced fullscreen with borderless fullscreen. Fullscreen exclusive mode does not exist in DX12 anymore.
Did you check the config file for the resolution (My Games\ArmaReforger\profile\.save\ReforgerEngineSettings.conf)?