Hm ..then something is broken, because when I set 540p resolution doing 2x SS, I get 1080p final image
Supersampling is what goes on 'behind the scenes'
If your display resolution is 1920x1080, then SuperSampling of '1' generates the scene at 1920x1080.
If your display resolution is 1920x1080, then SuperSampling of '1.5' generates the scene at 2880x1620. This is then downsampled to fit the 1920x1080 display resolution.
If your display resolution is 1920x1080, then SuperSampling of '.5' generates the scene at 960x540. This is then upscaled to fit the 1920x1080 display resolution.
Downsampling allows for a higher quality image, as it's better to throw away information, then try and make it up. However, your system is doing more work, so render times are longer (lower FPS).
Upscaling allows for a faster render time, as it's not doing as much work (higher FPS). However, the quality is lower, as the lower resolution image has to be 'blown up' to fit the output resolution, in other words, your system is making up pixels where no pixels exist. Just like your TV does when trying to put a SD or HD programme onto a 4K screen.
edit
If I understand your comment correctly, you have a 1920x1080 panel, and you're telling Elite to generate a 1920x1080 scene, throw away the information down to 960x540, and then your panel is upscaling back to 1920x1080.