They allowed Mazda to add a few more dot-points to the spec list in the brochure.
In all seriousness though, from memory the extra bracing was just another couple of braces around the area between the doors (under the car) and a strut brace.
Here's a pic of the underside of an SE (Mazdaspeed Miata since it's a US car)
being an SE it probably has some extra factory braces that the regular ones didnt, but gives you an idea of the OEM braces and locations.
Personally, the strut brace was not much use, I have one on my car and there's no difference. IMO it was just marketing (has a brace there, must be sporty!)
The rest of the bracing I can't speak for