The only thing is, cold air is more dense than hot air. That's why hot air rises. In my experience, there are just too many variables to try and really tell how the different temperatures affect a disc's flight. You've got bad footing, muscle stiffness, reduced grip, heavier clothing, stiffer discs, and denser air. All of these are symptoms of cold weather play, and it can be tough to tell which variables are having an effect on each throw.
If I had to break it down, I'd say bad footing, muscle stiffness, reduced grip, and heavier clothing all cause the disc to fly more overstable. I would think that denser air would make the disc fly more understable. As far as the stiffer discs go, I don't really know. It definitely makes them more prone to damage when they hit something, but shouldn't have a huge effect while in flight. So you'll hear all kinds of people making all kinds of claims. I say don't worry about it and enjoy the good weather when it comes (starting to get here where I live).