I recently sold my Tamron 150-600 G1. I had looked into updating it with the G2 but finding any reviews that actually straight out said that it was better at 600mm than the previous generation was nearly impossible. So dishing out the money for something that turns out not to be better in the quality department than the predecessor seemed pointless. I don't need a fancier lens for that money, I need more sharpness.
So after a lot of research I decided to skip Tamron altogether and to go with the Canon 100-400 mkII. I don't regret it one bit, despite the hefty price tag. I have the 7d mkII body and got the 1.4 canon extender with the lens.
I upgraded my firmware and added the lens profile and the profile with the extender. The results are beyond anything I could have ever gotten with the old Tamron. I've photographed a lot of BIF with the Tamron, and usually at the most 1 of 30 pictures were really sharp or just sharp enough to be acceptable. In comparison, I took the Canon for a walk, I had the extender on, and suddenly saw swans flying by about 30m away. I wasn't ready and hadn't double checked my settings, but I took a high speed set of pictures with it, using the centre focus point and from 30 pictures ONE was UNSHARP. The rest completely smashed any previous Tamron results, even with the extender!
So anyone thinking about getting a big tele lens for one of the higher tier Canon cameras (those which still can focus at f8), you will not get disappointed with the Canon 100-400, despite the slightly shorter reach!