Magincap
What you want todo takes VRAM and in Windows and Windows 10 that is a critical space Higher upscaling can only work if you have enough VRAM resources or if you Share the resources with other Memory parts which can compensate your VRAM running low but also slows down the possible rendering bandwidth significantly, result extreme performance hit shuffling the data from the Card over the PCI-E back and forth through the Main Memory to the CPU.
Since Windows 8/10 consume more VRAM through it's compositing engine (the nice UI message window you see there,telling you no go) you will run out faster of it as well and more needs to be shared.
Unreal Engine tells you that the Developer was not yet able to define a simple requirement measurement counter for this Feature (he can implement failing Protection measurements though) and show you a critical area you gonna reach and advise you it could fail like for example how Rockstar does it and now their support overhead grows for all the "lewdies"
Also test different Adrenalin there are reports that the latest is not always the best option depending on your AMD cards architecture, though it most probably wont help you (unless something has changed in the memory resource management) as you simple put running out of fast VRAM resources or even shared memory pool capabilities on your system overall to use this Upscaling Ratio.