Let Windows 8.1 use 5k pixels of my iMac 5k? (Boot Camp) So I installed Windows 8.1 x64 using the Boot Camp Assistant on my Yosemite, iMac 5K 27".

The max screen resolution is being reported as 3840x2160.

I cannot explain what exactly is wrong, but my strain is huge after working in this setup for ~2 hours. It wasn't so in MacOS. I suppose the problem is with graphics driver.

I updated Windows 8.1 - everything I could.

What worries me is that the latest Boot Camp Support Software 5.1.5640 does not list iMac 27' 5K:

https://support.apple.com/kb/DL1721?locale=en_AU

.. and the AMD website where I wanted to try their driver here:
http://support.amd.com/en-us/download/desktop?os=Windows%208.1%20-%2064

says:

This driver is not intended for use on AMD products running in Apple Bootcamp platforms. Users of these platforms should contact their manufacturer for driver support.

Then, I tried to contact Apple Support - I have an extended support for 3 years (AppleCare). They asked me nicely to go and ask Microsoft for this.

Finally, I contacted Microsoft Support and they proposed that I subscribe to their support service (e.g. spend another $100+) and they will solve my issue. They assured me they knew the solution and they even had an internal technical  note about a similar case. Besides the feeling of being cheated, I supposed there should be drivers for my case, but I can't find anything.

Please help me - how else can I try to solve the issue, what system info you want to know in order to give advice, and where should I look for more references / reading?

UPD. I've also tried these drivers from AMD designed for BootCamp, but it did not help:
http://support.amd.com/en-us/download/desktop/bootcamp
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?