I have the digital EOS rebel, the 6.3MP version of the xti. I I have never noticed any performance issues in cold weather, but have only had the camera down to about 15F.

I also have the same lens, and I am not sure about what you are describing. My screen displays pretty much the way it shoots. In other words the screen is the picture, but there may be a setting for that ( I don't have the camera in front of me to check)
FWIW I have not noticed a difference between that lens on the digital vs. the 35mm film camera I have, but I have not actually tested that.

When used with a t-adapter on a 130mm reflector I got great moon shots, at least for a very first attempt. With the digital that is.