Has anyone done work at around 20,000 feet (8000metres) or above with the 5D and how did it go?

20,000'? Wow.

I live at 8,500' and routinely shoot at 12-14,000', but not 20K. At this "lower altitude," the camera works flawlessly. You might want to make sure you have a good UV filter, however. At altitude the UV rays are more intense.

Your 5D should work fine, as long as you keep the batteries warm.

8,848 metres with 5D MK2 + 15mm lens (This photo taken at base camp, 5.180m = 16994.75ft):

http://www.trekearth.com/gallery/Asia/China/West/Tibet/Tingri/photo1254532.htm

Great pic and good to know - thanks