Before starting a legenthy post; if you have any other monitors connected, does disconnecting them fix it? Both port B and D should give you native boot resolution.
Assuming you're not running with CSM enabled (BIOS compatiability mode) then the boot resolution on NVIDIA cards should be the native resolution of your monitor. -- More specifically the boot resolution should be the first detailed timing listed in the monitors EDID chip.
The vBIOS will dump the EDID during startup (before anything is displayed), the UEFI will then load the EFI GOP module from the video cards ROM, the GOP module checks the timings and will try to negotiate a mode switch to the first detailed timing. - Assuming the UEFI supports that timing (resolution) of course.
I've seen some UEFI's that only run in 1024x768, 1080p, some scaled/stretched resolution or even only run in native res using the iGPU. I've also sees some NV GOP modules that are hard limited to 1080p max or won't even display anything on HDMI 2.0 (had a fun conversation with NV techs about the HDMI 2.0 one >_>).
I would try a different port (on the screen itself - each port has its own EDID), update your UEFI, update your vBIOS, update your screens firmware (if available) and make sure the CSM is off. You could also try disabling your iGPU and making sure that monitor is only connected with one cable.
As a last resort you could remove the card, reset your UEFI to defaults and then put the card back in.
Side Notes: Windows display drivers have nothing to do with GOP video modes. That can be the UEFI, the Windows boot animation or whatever. So there's no reason to mess about with display drivers. There is also no reason to install monitor INF's as unless ASUS messed up the EDID (and needs a EDID override) then the PNP monitor driver is perfectly fine.
post edited by Quad5Ny - 2017/04/27 13:17:03