When modeling my stuff I always wonder if it is really necessary to model low poly collisionmeshes. It is said you should for performance reasons, but usually you can just use the visible mesh and it works fine, there is no performance problem with that. Organic high poly shapes like player models of course should have low poly collisions, but in case of the player a whole other system is used with the bounding box.
So when does it make sense to use low poly collision meshes?
I made some tests and only really high poly objects give performance hit if you use visible mesh for collision, for example I made a metal grid that the player walks on, all out of meshes and with visible mesh collision and there was a noticeable performance drop when walking over it.
For regular scenery objects visible mesh works fine in most cases, also I could imagine physical objects that are rigid and/or destructible and fly around a lot profit a lot from as low as possible collision polygons.