That is an odd size. I just picked up an SE and just notice it. Hopefully there will be mroe choices next season.
For now, X-Ice has 235/65/16. According to
http://www.miata.net/garage/tirecalc.html, it's about 1% faster with this set. I did the same on my Altima as well when they don't have to size. These tires should clear as they are not much wider than OEM.
What get me more is the TPMS sensors need to be installed on the winter rim. That's at least another $200-$300 just to buy extra sensors for winter rims.