Image upscaling works when the image needs to be upscaled.
Yes, 600x400 to 1920x1080 requires upscaling.
No, 1920x1080 does not require upscaling. However, you can have madVR double the resolution and then downscale it again if you enable super-sampling with one of the doubling algorithms. I don't recommend this.
Chroma upscaling is a more complicated concept. Almost all consumer video has 4:2:0 chroma subsampling which means that FHD video has the luma at 1920x1080 but both chroma layers at 960x540. To convert to RGB the chroma and luma resolutions must match and chroma upscaling is what madVR uses to do it.
NGU is very good but best is not something we believe in here.
I would use madVR to upscale to 4K, assuming your GPU is strong enough, madVR's upscaling is very high quality with a lot of options to adjust it to be ideal for your eyes, projector, and screen.