Let Windows 8.1 use 5k pixels of my iMac 5k? (Boot Camp)
Back in 2015 there was a Bootcamp Update that came through Apple Software update (ATI/AMD driver), which enabled full 5k resolution for Windows 8.1.

Sadly Apple pulled this update silently. I only know the version number of that gorgeous driver: 15.201.2001-15005a-295390C

But the only way to get it, is a private person, which did that update and did not delete the installer, usually located in: "C:AMD/".

If I once get that driver back, I will share with everyone! Maybe someone can help?