You really botched that link up
Sport Edition SE-16 (not your fault, tire rack's link was super long and complicated)
205.50/16 is the stock size..usually best to still to those.
I suggest playing the differences in tire size here
http://www.miata.net/garage/tirecalc.html
+/- 2% is pretty safe and wont be noticeable but going down to 205/45 is closer to 3.5% difference and the tires will be pricier.
What suspension are you running? The main reason for going shorter over in diameter is for fitment when you are really low and that lower offset, et40, on a 7.5 wheel will be a little trickier too