Never handled the Tammy but when I was in Denali last year, my 100-400 was often too short. I did use it some with a 1.4x but now plan to return with the 300 and 2x someday. If I were in your shoes, I'd really consider the Tammy and bring the 70-200 with TC as well. My only reservation is that I was shooting the 100-400 at relatively high ISO's because of the lighting and the Tammy won't be any better.
Another alternative is to rent something long. A member of our group had a 600 mm on a crop body (with a monopod) and got some spectacular shots.
Thanks for the tips. I was wondering if 400mm would be enough reach, maybe with the 2x extender, but then I'm looking at f/11 and no AF. I have an EOS M can give me more pixels on distant targets. I'll look into rental options.