Still snowing in CO? Yikes. Winter won't let go, eh?
My 400 5.6 works very well with the Kenko 1.4x TC on the 6D. If the 6D had (or gets) Canon-blessed f8 focusing, I might be tempted to pick up a Canon 1.4x TC III. But there's no sense in doing that if they don't bring f8 to the 6D.
Nice shot, btw. Must be very nice having IS on such a long lens. That's the one thing that's tough about the 400 5.6 and 1.4x TC combo. Really hard to hold still with it.
"I know rocks. Rocks are hard."