Before I heard of the DX11/High-Res Textures I found Crysis 2 to run at around 39fps extreme and about 46-50 on very high, so I had a very good first play through. I uninstalled and then learned of these updates.

I only have the DX11 patch installed at the moment. With DX11 on I can tweak extreme/ultra/very high and get about 30 fps on the go.

My question comes in about tessellation mainly. That seemed like the most significant improvement to me. HDR post-processing is beautiful too, but tessellation kicks ass. What setting mainly controls this? Do I simply need DX11 to be on? And does Ultra need to be on, and if so, for what settings for the tessellation to kick in.

Also DX11 vs High Res, what would you pick? What runs better?

Any help would be appreciated.

Edit: I own Crysis 2 through steam, just incase that matters.

Yes there are sort of. There is a thread with a list of cvars, which ones can correct colours for DX9, and which ones can improve the frame rate for DX11.
Also there are threads dealing with some of the dx11 issues, such as frame cap, startup, and glitch of one of the postprocessing ultra features.
You may have to do a search for them.

High res runs better if you have enough ram, though the full benefit of high res is only gain with DX11 ultra.

Crossfire scalling can be improved with newer beta (warning), drivers for amd and nvidia. However they are risky as they are beta.

Shading and object detail are the main killers of performance yes. Hi-res textures are largely irrelevant if you have the vram and memory but DX11 features are the biggest frame rate killers. Extreme to Ultra will drop mid range cards down to 30fps, while very high shading and object detail is idea for midrange cards at 1920x1080.

Note that texture size and detail rely on vram and texture quality settings are irrelevant on today's GPUs, since it hardly affects performance.