gm still knows aero
the insignia ecoflex has a 0.26 Cd (0.27 for the normal version)
at the time of the calibra diesels could not compete with sporty petrol engines they where economical but slow and noisy. the vectra upon wich the calibra is based did come with a 1.7L diesel so technically it would be possible to make a diesel calibra. it must be noted however that most calibras had a 0.29 Cd rather than 0.26 (only 8 valve models)
due to more cooling. so a diesel calibra will likely have the same Cd as the vectra donor car.
it is of course sad that a car like the chevrolet cruze for example wich is comperable in layout and size to the first vectra only manages a 0.31Cd 22 years later.