[1] 2.6kg is really too light for a lens of those dimensions. And it's not exactly heavy either.
[2] You wouldn't buy a 105mm filter for it I shouldn't think. It should take drop ins, although I haven't scrutinised the specs.
[3] The 80-400 OS is an interesting lens, particularly if it *doesn't* contain HSM. Then the very curious question will be why not, and the inevitable comparisons with the Nikon would arise.
[4] Claim otherwise all you want, on Nikon cameras, Sigma's HSM is not as fast as AF-S. In fact based on the 80-200 AF-D, on a decent camera, there are hardly any differences at all.
[5] Unless Sigma have changed something, their FTM is a joke. More like PTM.
[6] As another option, the Nikon 70-200 VR with a TC20E would be a serviceable option, as well as being a more versatile combination. And offer faster AF, since that's what everybody seems to make a huge fuss about.