215/55R16 is an uncommon size for the SVT. I haven't heard of anyone using it. You may want to go to the tire calculator to get an idea about how it will fit.
http://www.miata.net/garage/tirecalc.html Much more common are the two factory sizes, 205/55R16 used in both 98's and 215/50R16 used on 99 and newer cars. There are also several of us that have 225/50R16.
It should not be a problem to have a larger size on either the front or rear until you need two more tires. It is a common practice.
Jim Johnson
98 SVT
225/50R16 BF Goodrich KDWS
