You're probably better of using Real-ESRGAN: https://github.com/xinntao/Real-ESRGAN. It's pretty solid and fast, even has portable executables you can just use as is. The upscaler that comes with stable diffusion might work for you, but I suspect it'll probably do a better job at upscaling stable diffusion output rather than a natural images (might be wrong though).
I tried this on one of my home images. I have a nice canon pro 100 printer that can print 13”x19” pictures, and my camera is a 20 megapixel Panasonic GH-5. The printer can print much higher resolution than my camera. So I did take one of my photos and process it with Real-ESRGAN to double the resolution (in each direction, so 4x pixels). The photo is a red barn with redwood trees behind it. It did well increasing the resolution of the barn. It made it look more crisp and bright. But there is an area with some trees in shadow behind the barn, and it lost detail there.
Anyway I think it would be fun to play with, just depends on the content of the image and the artists preferences. I still haven’t printed a full page of the upscaled photo but I do want to try that and see how it looks in comparison!
