Upload & Sell: On
As the matter of fact, I am considering going this hybrid way. Before D3, I was shooting 1DII for many years and loved the camera.
Intuitively I think the 400 f5.6---a prime--+tc 1.4x would give a much better IQ than the 80-400 f5.6---a higly zoom-- + tc 1.4x.
It's not a bad way to go if you have the resources to support two or more systems, and it's what I do for wildlife shooting and I love it. My favorite combo is the 7D with the 400L 5.6 for birds and other wildlife along with the Nikon D5100 or D7000 with the Nikon 12-24 f4 for landscapes. Sometimes I will instead take the 70-300L with the 7D and/or the Nikon 17-55 with one of the Nikon bodies.
I thought long and hard about giving up the Canon system when the 80-400 AF-S VR and D7100 came out but for now I think I'm going to wait and see what the 7D replacement is like before I make any decisions.