Can I run 205 width winter tires on the rear wheels?

 

Yes, you can. In fact, I recall another local owner having done this with his first S2000. However, that was with aftermarket rims all of the same width. Putting a 205 tire on the OEM rear rim might pull out on the bead just a little too much, so you would have to experiment with increasing tire pressure to better plant the middle of the tread.
It'll look kinda weird to see those skinny little tires out back but in the winter, who really cares?

 

xviper: Thank you so much for your clarification!! appreciate it!