@Vibstronium: Thanx for the advice. I usually think that native resolution is the best/sharpest.
BTW what exactly does render scaling do? Is it sort of up- (or here) downscaling done by the CPU to let the GPU ‘breathe’ so that it can cope with such a big resolution (4k)? And how does VR fit into the picture? It seems as if resolution and fps are set by the glasses’ capabilities then.