I was thinking that the Rx-8 should probably have 215/50 17 as its snow tires, if it has 13" front rotors.
215/55 16 would work on the 12" front rotor equipped cars.

And after the OEM Bridgestone Potenza RE040's wear out, I would replace the 225/45 18 with 235/40 18 tires. A little more appropriate for an 8" wide wheel.

Comments, please.

 

*Shrug* Not being a winter driver, I can't help you much here. It is worth noting, however, the Bridgestone Blizzak tires are available in the standard 225/45R18 size, so you don't necessarily have to ditch the 18" rims.

 

winter driving implies marginal conditions.
the extra rubber between you and the road comes in handy.
and in heavy snow the narrower tires will handle better. so i would go with the smaller rims too.

not to mention that trashing 18" rims costs more than replacing a 17s or 16s.