215/45s would be ~3% smaller diameter than the factory 215/50 x 17"s, giving an actual 58 MPH at an indicated 60. You would also have a slight error in apparent fuel economy and burn through your warranty 3% faster (i.e. 300 miles lost every 10,000) Some might say this is too big an error, but I've driven lots of cars with much larger errors in the factory speedo calibration and I'd say go for it!Check out this site for details:
http://www.miata.net/garage/tirecalc.htmlThe extra 1/2" width is no problem, it's more important that the offset is within say +/- 5 mm of stock. (I think that is ~ 38mm, can anyone confirm?)