I've been worried about the heat in a Passion, too. I live in Vegas, and it gets pretty hot and sunny here. Not only is it "Mostly Sunny" nearly every day, but in the summer time, we have sun up until about 9:00pm.
For the most part, when I finally get my Smart, it will be parked in an underground parking garage during the day. After work, though, it will be parked on the side of my house (I will already have two other cars in the garage). I don't want the sun to bake the inside of my Passion through the roof, and I've been told by many people that tinting the roof is not a good idea. I'm bummed, and I don't know what to do about it. I'd really like to find some sort of stuff that could cut back on the light just a bit without clouding the roof.
As far as AC goes, I used to have a 280Z with no AC. I just kept the windows down. I'm a native to Vegas (very rare breed), and I'm pretty used to the heat. AC gives me a headache, too, so I can't have it blowing in my face.
I'd love a Cabrio, but 1) I can't afford it and 2) I'd only get about 2 good months of top-down weather.
As far as humid vs. dry, AC doesn't work as well in dry weather. The moisture of humid weather actually cools the compressor down more, which coupled with the movement of the air during driving, causes the AC unit to be able to cool better. France may get hot, but it's more Washington State hot than it is Vegas hot. Even so, I was impressed with the cooling ability of the Cabrio when I went for my test drive here, so I can't imagine how cold it would get in the more humid regions. But then, 78°F is cold for me.
Also, humid heat sucks, but humid shade is much cooler than dry shade. It's easier to escape humid heat, which is why everything in Vegas is dead. It's not easier to escape humidity, though, so don't mistake the two. And most AC units will only cool to 30° or so below the outside temperature, so 120° outside means at best 90° inside.
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