Generally, it's harder to improve on a car with a lower Cd, because the low hanging fruit has already been picked.
FYI:
Info & video - Opel Calibra - Cd 0.26
Despite that, there's probably still some improvement to be made:
- remove a side mirror or two
- potentially further grille blocking (though the 8v Calibra already has a reduced grille area)
- rear wheel skirts
... Have a look here for anything else that may apply from the
List of aero mods you can do to your vehicle.