Seeing Skyrim shots right after Witcher 3 shots has me convinced people who think even modded Skyrim looks better than Witcher 3 are insane.
Retail Witcher 3 does look different from E3 2014 Witcher 3, and mods can add a lot to Skyrim, but no amount of modding can cover up the Xbox 360 level of detail upon which Skyrim was built. It's still clear the two games were designed for two different generations of consoles. Similarly, go look up videos and screenshots of modded GTA San Andreas. You can change the resolution, you can change the textures, you can change the lighting effects, you can extend the draw distance, but you can't upgrade the PS2-level polygons to look like GTA IV or V.