I downloaded the install files for the Witcher 2 from GOG.com when it was still at 3.0.1.17. I tried to reinstall today due to some issues I was having but it turns out I'm missing one of the bin files (setup_the_witcher_2_ee_3.0.1.17-5.bin). The older bin files don't seem to be listed on GOG anymore though. I'd rather download a 2GB file rather than the whole 16BG again if possible.

Try contacting support. Perhaps they still have the old files backed up and are willing to give you a link on one. Otherwise, no, there's no official way to obtain the older files.

The Witcher 2 download sizes were already annoying, now they're unbelievable.

Well, that's the price you pay for high quality assets.

I suppose the game is large and has good assets, but among CD Projekt RED's many qualities I don't see efficiency, just look at the terrible performance of both Witcher games up until the enhanced editions came out, so allow me the benefit of doubt.