Canon has just done that. A new lens just as sharp as the old, and they have added a zoom down to 100mm as well. It also closes up small for ease of packing.
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The Tamron at 600mm and f/8 on the 5DIII is as sharp at the centre as the 100-400 II at 560mm with a 1.4xTC."at the center" is the key word here. Comparing the corners gives a completely different picture:
From what I've seen and read, this lens is really just a 150-400mm lens. 400 and beyond is soft and the Canon 400mm 5.6 upsized to 600 and even 700mm is sharper at the Tamron's native 600mm setting! For me personally I'd get the Canon 400mm. And I plan to eventually.
This is a bit disappointing (especially since I have one on order), but not entirely surprising.
Focal lengths are routinely rounded up (and often times down on wide angles). The 70-300L is noticeably shorter at the long end than the 300L F4 prime. It's quite obvious.
It's disappointing but, I suppose as long as the 100-400 II is 100mm longer than the 70-300L, then that makes it worthwhile.