I'm surprised you've had to take it down to normal, I'm not sure what the GTS 250 is like but my 4870 1Gb is happily running it on very high at 1920x1080 in Dx10. i5 at 4Ghz and 4Gb DDR3, can't see your processor or ram being the culprit.
It runs very smooth as long as those massive beams of lights don't make an appearance, fortunately they've only shown up at the start of two levels briefly, then it's back to smooth performance.
I didn't play the demo but the story/characters of the game are alright so far, Bourbon is a pretty funny character. A fair bit to interact with in the game too.
Dx11 plays a very minor role in Metro too, not worth the performance drop as the tessellation isn't very good, not really that noticeable unless you stand still and zoom in on object edges.