I have 3 monitors: a 3840 X 2160 (4k), a 2560 X 1600 and a 1920 X 1080
Mostly I play VG and now iS on the 4K monitor, but I've done some experiments on the other two.
The "natural" size of a 1080p card on a 1080 high monitor is about 72% of full screen. If you play a UHD card at, say 90% on a 1080 monitor it wll look better than a 1080p card at the same 90% setting because the software will be calculating the display pixels from the higher resolution card and not trying to scale up from the lower resolution card. However, this calculation will take more computing power, so a low powered CPU/GPU may give better results with the 1080p card.
If you play a UHD card vs a 1080p card at, say 60% on a 1080 monitor both cards will be scaled down. It's possible that the UHD card may still look better, but depending on the graphic compression the difference is likely to be negligable. The processing load between the two will remain significant.
Playing a UHD card at 100% on my 4K monitor is impressive, but I can see some fuzz around the edges. Rotating the 4K to portrait mode and setting to 100% is awesum, but you can definitely see blurring at the edges. I've only done this once as an experiment. The models tend to walk off screen a lot.
Yes at 100% on the 4K the top of her head is about at the top of the monitor in either mode and this applies to 720 and 1080 cards as well, but with much more bluring. The bluring of 1080 cards is not too bad. The blurring of 720 cards is so much that I seldom play them at more than about 40%. At one time Totem was remastering some of the newer 720 cards to 1080. I hope they resume this. I've got a lot of 720 cards I seldom play now.
Mostly I play single cards at 70% on the 4K to give me space to work.