You forgot to multiply by two... You only did one sidewall, the radius would be 1/2" shorter, the diameter would be 1" shorter.
Again not a big difference. Oh and that miata calc sucks...
http://www.1010tires.com/TireSizeCalculator.asp
Yea, I wasn't meaning that he would notice any gains, just that theoretically, the lowered gear ratio supplied by the smaller tires would give a little mechanical advantage.
