bah, screw the street tires, you can get 275/40 17s Hoosier A3S05s for the same price as the RT-615.
https://www.hoosiertire.com/discontinuedtires.htm
or 275/40 17 Z211s for a little bit more…
http://www.frisbyracetire.com/z211sale.html
same rolling diameter as 245/45 17s
http://www.miata.net/garage/tirecalc.html
and stock rims are $300 on ebay…