There are several comments here about the Tamron being soft at 600mm and slow from individuals who clearly don't have any first-hand experience of the lens and just rely on mythical hearsay or lens test from TDP, which actually shows it sharp at the centre.
I regularly use the Tamron 150-600, have used extensively the 100-400 L and have the 300/2.8 II.
The Tamron is not soft at 600mm, and it is reasonably fast at AF at 600mm. My first hand experience fits in well with the extensive reviews on ePhotozine and Lenstip. Here is the MTF analysis from ePhotozine, which has measured the values at 600mm to be on the edge of excellent at f/8, and the Lenstip's of the 150-600mm and the 100-400mm, which shows that the Tamron at600mm and f/8 to be similar to that of the 100-400 at 400.
Don't knock cameras or lenses based on hearsay, second-hand and inaccurate information.
Good stuff and there is a quite complimentary recent review of the Tamron on the photography life site.
Apparently the new 100-400 looks likely to be a scaled up version of the Canon 70-300. What a shame - a scaled up IF version of the 70-200 would have been better. Canon are cleverly pitching the new lens as not to be to sharp that it impinges on sales of other lens up the chain.
Unfortunately the extending telescopic lens design @400 could mean that even slight knocks will damage optical alignment.
TC capability is unlikely - how many crop cameras can make use of that!!
However I have to admit that the new 100-400 has a few things going for it - IS2, relatively light weight and Canon quality control assuming it is made in Japan but I think it will be expensive like the Nikon equivalent so I will stick to the current model.