Not quite the case you're after, but one that does point out the value in getting the rear shape correct.
A Pontiac Solstice Convertible has a Cd of .44 or .45 with the top up (hard to find "real" numbers). The Coupe version is 10% better, say about .41 (again VERY hard to find good numbers).
Top up side view doesn't really show what it's like:
I've been told the airflow never reattaches on the convertible, but that the tail on the coupe is just high enough to get some reattachment. (Note that they both use the same door glass, rear fenders, tailllights, and rear bumper/fascia.)