Yes, I would love that lens too, but, and this is a big factor for me, at almost 3 lbs heavier and $9,400 more expensive, it's a show stopper for me on two points. I'm always on the move & I prefer to hand hold and the weight of this sucker would basically require a tripod for me. I can't move nearly as well and I haven't developed a technique shooting with a tripod that is effective, at least for me. I really like the extra reach I get with the 600mm with a lens I can hand hold.
Unless the birds are flying, I'm using at least a monopod.... my tripod gets a lot of use too but I lug my gear around in a Rugged Gear cart... so I don't have to carry it to and from the birding sites. Still, the 200-400 is beast, as you suggest. I've been pretty happy with the results of the 1D X2/100-400ii/with the 1.4X TC, but it's at f/8.
I'll probably pick up the Tamron eventually because I've seen in just under $900 a few times now.