@Vibstronium: Thanx again, I see much clearer now.
Actually I checked your example flying in dev mode. In a given area with FullHD / 100% rendering and about 60 fps I obtained 45 fps with 4k / 50% and below 30 fps with 4k / 100%.
But the sharpness gain is sometimes hard to tell, at least at the scenery 
It’s easier to notice looking at a small font (eg. ‘VOL’ at the glass cockpit frame of the SR22).
Well, does anybody have some experience regarding AMD’s image sharpening function inside the driver?