What extra weight?
2009 Volkswagen Jetta wagon = 3228 lbs
2009 Volkswagen Jetta sedan = 3230 lbs
2009 Volkswagen Jetta specs, auto safety at Edmunds
2009 Volkswagen Jetta specs, auto safety at Edmunds
Drag Coefficient of .31 for both Jetta sedan and wagon.
http://www.cowellvw.com/jettawagonfeatures.pdf
Same thing happend with most Volvo sedans and wagons. There is essentially no aerodynamic drag penalty for the wagon.
Which is pretty pathetic considering the 1998 Passat had a CD of .27 for the sedan and .28 for the wagon. Even a 1994 Volvo 850 wagon had a CD of .31. So much for progress.