Schultzy,
The BBS rims fitted to your car were fitted to a small number of MX5s in 1993.
http://www.miata.net/faq/wheel_weights.htmlThe BBS brand is very highly regarded and those rims are top quality. They are a German brand. If you decide to sell them you should get a very good price. They are a 14x6 size, not the 15s (look at the lip around the outer edge). As pointed out, they are very light, the MX5 suspension gives it's best with light rims.
The fitment of a 15x6 rim to your car will not break the ADRs (nor would a 15x7), but you should check with QLD motor registry to find out their interpretation of the ADRs. From memory, 15x6 will be both legal and insurable, but you should satisfy yourself about that.
As for fixing the centres, you can try shimming the caps a bit (thin foam to start with) but BBS rims in that style were notorious for the centre caps. They were also offered on BMWs a lot and it was a major gripe. Too much shim and the centre will fall off, and replacement is very $XXXy.