I've ran 15s on 2 Micras and now I'm running 13s on my 1.3 SR... Honestly doesn't make that much difference. Tiny bit quicker with the 13s. You won't notice much of a difference at all with 15s over 14s, I imagine.
look at this site http://www.miata.net/garage/tirecalc.html
put in original tyre size and compare against the new tyre size
it will then tell you the difference that will be displayed on the speedo
i wouldn't recommend 15inch tbh i am sticking to my original alloys which are 13 and i love them when it come down to me needing some alloys i will deffo be taking 13 max 14