Being Norwegian, there are plenty of opportunities to test low temp performance during our winters. All of my current and also my previous bodies have been out at temperatures below -25C (-13F). Some also below -30C (-22F). That includes 5D, 5DII, 5DIII, 1DIII, 1DsIII, 1DX and probably also the 7D (don´t really remember). I have only used L-lenses. The only precaution is to pack the camera and lens in an airtight plastic bag when I go back in, which is something I learned back in the seventies and just continue to do. I´m not sure that it´s necessary for a pro body/L-lens combo, but I am not going to test the alternative.
The only problem I have had was with my AE-1 Program, probably around 1980. The shutter curtain froze. Apart from that, I have not had any issues with neither bodies or lenses.
Attached is an early January image from the mountain area where I have my cabin. It was -32C (-26F). 5DIII w. 24-70 f2.8L II.