I think I understand what you mean about smooth, but I think it's not the role of Sweet Home 3D users to decide whether smooth normals should be computed or not. It's in the imported file that precomputed normals or smooth parameter should be set. For example, smooth is set in OBJ format with a s selector followed by a number (and not off value). If this is the case, Sweet Home 3D will compute smooth normals.

About luxrender, did you try photo creation tool in Sweet Home 3D? What would be the difference between this tool at its best quality and luxrender?