I was wondering why people don't plump for 15x7/195 frt and 15x8/205 rear. Apart from not being able to rotate the tyres would there be any downside re mx-5.

Or alternatively there may not be any difference in handling and ride otherwise people would have been doing it for their road/track cars for years.
I must admit to a certain curiosity and it would probably look good.
Lotus and BMW have been staggering widths for years.
This is probably too extreme, but you get the picture.
