Last time I wanted to change to a new tire size, I was told by the supervisor that changing from the OE spec was not recommended.
I'm currently deciding on changing from stock 215/45-17 to a narrower/taller 205/50-17. The 10mm width difference and extra sidewall won't be too noticeable mechanically, nor have much fitment issues.
(Calculator provided by miata.net)
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/45-17 3.8in 12.3in 24.6in 77.3in 819 0.0%
205/50-17 4.0in 12.5in 25.1in 78.8in 804 1.8%