It will be fine, it's just like playing games at 720P on a 1080P monitor.
That's the opposite, that's upscaling, taking a smaller image and scaling it up. He wants to downscale, taking a bigger image and scaling it down to fit a smaller res. But it's similar in that it's not the native res being used which makes no difference to the monitor. In it's most basic form, downscaling is the same thing as supersampling. It's also what nvidia dsr does. You still only see 1080p as that is physically how many pixels are on the display but it could improve the image.