I tried out DSR as per https://www.youtube.com/watch?v=gSzgeyuyblM

In brief:

Using NVIDIA 3d global settings

1. Activate DSR by selecting the 5k resolution in legacy
2. Set the DSR smoothness level - the higher the resolution, the lower the smoothness you'll need so I went with 15%

Using Windows Display Settings:

1. Select the 5k resolution. The screen will flash on and off a few times then all is good.

You can pick a bit of a difference and there is only a slight hit on FPS, but I think I'll turn DSR off and stick to native resolution of 2k.