Words from FCP Groton parts house:

"The rear calipers on these cars differ by the piston diameter, there is a marking on the caliper fob with 36,38, or 40. This will be very difficult to see, but to get around this, you can order 2 calipers with the (40) diameter and you will all set if you replace them in pairs."

If you calculate the difference in piston area between a 36 and a 40, you could see how it might affect braking bias between front wheels and rear.

If it were me, I think I'd pull one of the shims out and measure the imprint on it.