Detaching objects in 3D program as you've indicated in the past is usefull for making unique vertices (with different normals) for a plane, allowing for hard edges. But if you import it as a single object, it's still one object, just with a little bit more (duplicate) vertices. So in context of what I wrote above it won't make alot difference as far as performance is concerned.
But if you import the subobjects from an OBJ-file into seperate 3D chunks, then they are considered seperate objects that can each be loaded (or unloaded for that matter) whenever needed. When an object is loaded into system mem but can be clipped because it's outside of the view frustrum, then it won't pass through the entire render phase, leading to better FPS. Big objects that are only visible for 10% still need to pass through the render phase completely, which hurts performance, especially when you run low on GPU mem...
So yea, in some way the developers definately considered this, although it's not apparent (or not needed) in all game files.
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