1. Install the high res textures pack.
2. In the Patch folder in your Crysis 2 directory, copy the TexturesHighRes.pak file into the "Gamecrysis2" folder
3. Rename Textures.pak to something like TexturesBackup.pak
4. Rename TexturesHighRes.pak to Textures.pak
5. (Attempt to) play the game! Epic memory allocation errors, memory spikes and computer freezing! Great fun

Cmon someone's got to try this

If point 5 is just a guess, then I'll go ahead and say that the game won't work.

As for why, then the reason is that TexturesHighRes.pak doesn't contain all the textures.