nice car!
i just bought a vectra a hatchback...on wich the calibra was based...it's "only" got a 0.29Cd...and a far less exiting, (but possibly more economic 1.6i engine) but i'm planning so mods like an undertray...
this video has an interesing fact about the calibra around 3:15
aparently the 16v versions had a much higher Cd... up to 0.29 because the upper grill was open....and it has wider tires, so blocking the grill would have a rather large effect...
also if the upper grill is already blocked you might want to fair over the opening at the outside, that will smooth things out further.
do you have a picture of the undertray? i've been looking all over the internet but i just can't find a single picture of the thing. i wonder if it would fit on my vec, or at least find out how it attaches so i could build something similar.