Thanks Slin.

Regarding the polygons: The corridor you are mentioning has some splits at the ground for using 3 different floor textures. So it is not one big plane made by only 2 triangles, there are some more cuts for uv-mapping. And as I already mentioned there are backfaces for everything even for the columns to create closed meshes. This is needed for stencil shadows (C4 uses stencil shadows).

I also preferred the C4 scene, but now I changed the materials in the Unity scene a bit to look very similar to the C4 metal while still using more lights. I think it looks better now.

The shadows are dark. In my earlier versions I had a brighter ambient light, but it did not look good with the flash light. So I darkened it again. And I personally love high contrast between dark and bright. And ambient light washes everything out. Normals and spec are less visible and there is no mood then.
But I will also try the lightmap render functions in Unity and UDK to look how it feels with some bounced light.