Since upgrading to a GeForce 750Ti and fixing a problem on my setup, I can now play Batman AC in DX11 mode acceptably (though there is the occasional frame rate drop). I'm just wondering though, does anyone think that the game in DX11 mode is so much better than in DX9?
I wouldn't say it's "much better." Arkham City was Rocksteady's first stab at DX11, and all they used it for were some extra IQ effects, like contact hardening soft shadows, HBAO and tessellation. The contact hardening soft shadows were only for characters I believe, so that didn't really effect the overall look of the game. HBAO is better than SSAO, but ambient occlusion is a subtle effect to begin with.
The tessellation was used on quite a bit of surfaces on the other hand, and it did noticeably improve image quality to the point where I'd say it was worth it to enable..
But the biggest IQ increase came from PhysX. Arkham City is one of the best PhysX games of the old generation, but I don't think your GTX 750 Ti would be powerful enough to run it with the other effects enabled..