Best way to reach 800mm f/5.6 ?

For bird photography, what solution would give the best IQ ?

1) Nikon AFS 400mm f/2.8 VRII + 2X TC III
2) Sigma 300-800 f/5.6 HSM DG EX
3) Nikon AFS 600mm f/4 VRII + 1.4X TC II

This exercise is part of my present reflexion about long-lens realigment.

Thanks
N.B. I have the necessary tripod and gimbal head for such focal lenght.