The fog in INS is a consequence of large maps and engine limitations. The reason you don't experience exactly the same issues on HL2 maps is because the view distance is blocked by buildings, props or the terrain, hence never truly seeing the limitations of the engine. When we designed INS we did it looking at pushing the boundaries of the source engine, and thinking that future versions of this same engine would provide performance updates. INS will reach a much better graphical level once the transition to the Orange Box engine is complete - and that will allow us to prevent all the reported draw distance issues and overcome all the limitations of the current engine version we're using.
Until then, there's nothing to do with the draw distance. Given the version of Source we use, there's little or nothing we can improve on that field.
The sole reason why you don't see the same effects in all INS maps is because smaller maps do not run into the same limitations. Additionally, some level designers have chosen to use the same tricks used in HL2 maps (placing buildings and scenario props blocking the view - preventing you from running into the limits of the draw distance).
Any time we try to represent a large outdoor map in this version of the engine we have the same draw distance limitations. Good luck finding other large outdoor maps in HL2-based mods. All of them will have the same issues.
Be patient. Good things will come for those who wait