I think it is the best way to save memory and bandwidth. But nowadays having 8Go of RAM or more is common.
If you target mobiles it sure can help these devices.
I assume it is something not far from what I did to generate normals from textures automatically. (better than not having one with deferred)
it talks about powerof2 resize, but it is I think a good start.
You could also set the mipmap level to be mapped to the quality the user chooses, and use this value.
But I suddenly have a doubt, I'm not sure if the mipmap is accessible with shaders in monogame, I remember having read about that somewhere here...