I ran the Battlefield 4 Beta on Windows 7 64-bit (DirectX 11.0) and Windows 8 64-bit (DirectX 11.1) and I can confirm that DirectX 11.1 DOES provide very noticeable benefits both in frame rates and lower stutter, etc. It felt like my processor was simply being used more efficiently. I was getting 50 FPS @ 2560x1440 Ultra settings initially (W7 DX11.0). I was dissappointed because I spent $1,300 on SLI GTX 780s and wanted to make use of my 120Hz Korean 2560x1440. In W8 (DX11.1) I was getting more like 70-80 FPS. It was really noticeable on my 120hz Korean monitor.
Keep in mind, that was only when all my games weren't crashing. And I mean everything was crashing, BF3,BF4,Far Cry 3, Blacklist, Steam, Origin, in Uplay...everything was crashing. Sad Face Screen of Death, freezes, Memory faults, driver faults, raid 0 array faults, unpaged_area faults. Windows 8 and drivers are like oil and water. It wouldn't even let me upgrade from a fresh reformatted copy of W8 to W8.1. Junk. I went back to W7 just so I could play at all.
Here is my system:
Software:
Windows 7 Ultimate 64-bit/Windows 8 Pro 64-bit
NVidia (Tried both 327.23 and 331.40 (beta now) in both W7 and W8)
Battlefield 4 Beta (@Ultra Settings minus Ambient Occlusion/Blur), BF3
H12ardware:
i7 2700k @ 3.5Gz/4.0Gz
QNIX 27" 2560x1440 120hz
p8z68 Deluxe Gen3
EVGA GTX 780 SC SLI
16GB RAM
2 x 128GB SSD RAID 0
UPDATE: Now that Battlefield 4 is out, I have the R331 (331.65) drivers from NVidia and in W7-64bit I'm getting decent frame rates: 0Avg ~65-70 @ Ultra settings 2560x1440 (Minus Ambient lighting, and only 1 of 4xMSAA or Differed HIGH). I do see large variation in framerates and I know DirectX 11.1 REALLY helps with that so if no one else is having issues with W8/8.1 than I'll switch to it soon since I have the discs already.