its because the 45 stands for the percentage of the width of the tyre not the actual measurement of the height of the sidewall, hence 225/45/17 giving a larger radius than 215/45/17 :)
i got offered 10.5k for my 08 cup on 22k miles by my local dealer just the other week, that was against a new 200.
was suprised at the offer but he wouldnt budge much on the price of the 200
same here, if you go into a corner fast and smooth you can feel the car slide slightly without the traction control interrupting, but if you throw it in the traction control goes mental.