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Just a question about your wireframes: Your basic wall has 3 polygons. How does it render with "smooth polygons" enabled? To switch that off for the entire scene is inadvisable if you have other, organic models in the scene ;)

Hello,
that should not hurt as long as the vertices are not welded.

In blender you can assign an "EdgeSplit" modifier to objects. That prevents smoothing
sharp edges. If you export with or without does not change the number of facets,
just the number of vertices and edges.

Best regards,
    Michael