OK I finished it. And you are right. I can see no difference when I am a distance away.
Here is a example as to how good the new textures look like after 2x scale. This was a original 256px texture from the asteroids (I think the smallest ones) that is now 512px. The left one has been scaled up using nearest neighbor technique, giving a better result than methods like Lanczos. The right one is scaled up using waifu2x, which results is an amazing quality. It's magic!

Incredible!
In game screenshot, original:
https://i.imgur.com/HymzKGp.png
modded:
https://i.imgur.com/Hn1XXwr.png
Note: Only the asteroid is changed. I have not sharpened any images, just scaled them up. I made the mistake and made DDS files from PNG files (though fully compressed), and so the mod is 172 MB big. xD
Quite stupid.
I think for such big things like asteroids the textures should be even bigger. I think I'll make a 4096px version, slightly sharpened, that is way better compressed. Just to waste some time.
Now I have a good workflow, thanks to GIMP and BIMP. It shouldn't take nearly as long as before.
This was my first time making mod for any game. xD
*Pictures greater than 640x480 linked. See forum rules - N8M4R3*