I did this last spring with a rented 5D III and Canon 14L. Did it last fall with a 5D II and Canon 24L. Now just did it again with a 6D and Samyang 24(as well as the Samyang 14 and Zeiss 21).
The 6D is simply a beast with high ISOs and the night sky. Even more of one than the 5D III was, easily trumping it. 12,800 was not a problem at all. Quite amazing actually. 10,000 felt like one was starting to push it for the 5D III. The same feeling probably comes in around 20,000 ISO or so on the 6D.
The Samyang 24 F1.4 was a load better for this than the coma abusive Canon 24L II. Plenty fine using it at F1.4 for this. Paired on the 6D it is pretty insane what can be had.
The Samyang 14 also a lot better for this than the Canon 14L II.
All images on here just one exposure, so no tracking stuff obviously. http://www.extremeinstability.com/2013-5-4.htm
I made a second page with full size crop examples for some 6D high ISO ideas as well as the coma of the 4 lenses. I could probably add something if something was wanted. It's clearly not a direct comparison as I never shot with that in mind and had the gear at completely different times. It is what it is and I'm sure enough to give some an idea what to expect that might help. http://www.extremeinstability.com/2013-5-4b.htm