Also take into consideration other aspects of your scene such as cameras and lights.  By default, they are checked as visible and are very much included in the render process although you don't physically see them according to the manual.  Systematically shut those down before render.  Again, any object/element switched to invisible will not be under render consideration.  

For distant models/props try using Normal Maps for details; this process is used successfully for lores backgrounds in games without significantly increasing the memory overhead.