The 4k series don't even support vanilla win8, not to mention 8.1... Upgrade your card or stick to win7.
I play Splinter Cell Blacklist and Grid 2 without installing any AMD Catalyst driver and it works surprisingly well
change in DX requirements for newer games? What do you mean exactly? Games use whatever DX level they were programmed with, as long as the OS supports it. A game requiring DX11.2 won't work on Vista, but a game requiring DX9 will work on Windows XP, etc. Games that have multiple DX levels use the highest one that the hardware and OS support, unless the user specifies otherwise.
Origin works flawlessly here - I have four games that I bought/acquired via Origin, and all are absolutely flawless in 8.1 RTM (they were flawless throughout the Preview as well) - SimCity 2013, Burnout Paradise, NFS Most Wanted, and NFS World. Steam (same desktop) is also flawless - in fact, I reinstalled Tactical Intervention last night. While some games on Origin (and the Origin client itself) require .NET 3.5, as long as you have the ISO mounted, you're fine (identical issue applied to Windows 8). I have not had any OS-specific issues with anything - hardware or software - as far as 8.1 has went.
Saints Row IV stuck at 64 fps, vsync is disabled but the game acts like it's on. On Windows 7 I get 90-100+ fps. In Windows 8.1 its locked steady at 64fps. I had these issues with Crysis3 when 8 went RTM. DX11 in Win8.1 has problems with capping around 60 fps like vsync is enabled. Disabled in game and in nvidia CP.
Could be tied directly to DX10/11 (depending on the highest your hardware supports). I have a DX9 game that goes way past 64 FPS easily, while fullscreened. I'm on AMD though.
What GPU? If you have AMD HD4xxx or less or nVidia GT/GTS/GTX4xx, no DX11 or above-only games (includes Crysis 3 and FIFA 13) will work in any version of Windows (including Windows 7), due to lack of a fallback to an earlier version of DX in the game itself. Crysis 2 and FIFA 12 include such a fallback - which is why they still work; same goes for Burnout Paradise.