Pinhedd :
Metro 2033 uses DirectCompute/OpenCL for its depth of field filter (which look gorgeous by the way). This means it has a massive slant towards AMD 7000 GPUs because NVidia castrated the Kepler architecture in this regard. To compensate for this many reviewers don't test Metro 2033 with Depth of Field turned on (some mention it, some don't) and the 670 is still a bit behind the AMD cards. With DoF turned on and AA turned on, it's far behind and basically unplayable.
Witcher 2 has a mode called "Ubersampling" which renders multiple frames and interpolates them. This differs from Supersampling which renders a single frame at a high resolution and interpolates it. Ubersampling looks gorgeous but cannot be run smoothly at high resolution on any hardware, not even 2 overclocked 7970s.
Really, Depth of Field on Metro 2033 really makes a huge difference if turned on or off.
My GPU is an "old" GTX 570, and I usually get +10fps with it disabled when playing on 1080p with everything maxed out (using AA instead MSAAX4).
With DoF enabled, sometimes I get annoying fps drops, around 20-24 fps.
But with a resolution of 1440p, even with the with a 670, I think it would be hard to keep a good fps all the time with everything maxed out.
1920*1080 = 2073600 pixels
2560*1440 = 3686400 pixels
1.6 M more pixels for the GPU to deal with.