another thing: use LOD, you can also create your own system with some lines of c-script (seein the last aums) for far away models, you can also use models with just 10faces or a sprite.
also you could use vec_to_screen to locate if the model is in the screen, if not make in invisible

here a little example(play with it and youll get good results)

Code:

---

action low_fps_model
{
  var my_lod=1;
  while(1)
  {
    if(vec_to_screen (my.x,camera)==0)
    {
      my.invisible=on;
    }
    else
    {
      temp = vec_dist(my.x,camera.x);
      if(temp<500&&my_lod!=1)
      {
        ent_morph(me,"high_poly_model.mdl");
        my_lod = 1;
        goto(end);
      }
      if(temp>10000&&my_lod!=3)//far far far away
      {
        ent_morph(me,"sprite.tga");//make me 2D
        //here parhaps a scale...
        my_lod = 3;
        goto(end);
      }
      if(temp>5000&&my_lod!=2)
      {
        ent_morph(me,"low_poly.,mdl");
        my_lod = 2;
        goto(end);
      }
      //you can add more stages of LOD,but you have to make more models ;P
    }
    end:
    wait(1);
  }
}

---

edit: because the skycube: you can try to make the sky-color near to the fog-color(think this is also possible with scripting), just look at AAA-games, they all have this problem.