This is not possible as the only downscale options are 75% (1620) and 50% (1080). Other games you can downscale to any resolution the monitor supports. Why is this limited to 25% increments in BL3?
I know it seems like a minor change from the 75% option, but it pushes my rig into the stable 60fps area.
Also… fullscreeen 1440 is not stable when alt-tabbing back to 4k desktop and does not perform as well as even 1620 borderless.