The file's arent further compressed, the models are in lec's 3do format. There are already some tools that can edit 3do's as used by (Jedi Knight?). In Grim though the format was changed from text to binary and some things moved around.
The models in the demo version of Grim used the original 3do format though, so you can open them with existing tools.
To increase the poly count you'd have to edit the model, add in more detail and then save back to the original 3do wouldnt you? You might be able to do it with the 3do's in the Grim demo but not with the ones in the full game unless someone writes a new plugin for editors.