I think we need to chill here - the aircon pumps from ye olden days were massive things akin to washing machine motors the newer stuff is far more efficient.
The Zed remember has a 3.5 Lt V6 - this has massive torque (like a diesel really)- it is not a Nissan Micra 1.0 engine. If you have the aircon on all the time then yes it will help to drink some of your petrol more so around town driving, on the motorway I not very much.
So dont worry too much about the aircon being on -it will hardly make that much difference to your fuel bill, well I never really noticed much difference when I had my ragtop and as Mart says Ragtop owners tend to fog up quicker than tintops.
In fact it was too damn efficent and I used to switch it off once the snowman appeared in the other seat!