When I import the OBJ file you shared, it's all gray because the textures aren't applied correctly.
I loaded the SKP file, exploded it, and applied the "specchio" mirror texture to both faces of the individual mirror tiles. I then exported it using File --> Export --> 3D model. Note that this produces a DAE file plus a directory with the same name containing the 4 textures in your design. The DAE file can be imported into SH3D and should be in the same directory as the texture directory.
This is the image that your design gives after importing into SH3D. I guess this is what you wanted? Note that the "specchio" texture doesn't result in mirrored surfaces, as the image clearly shows.
Hans
