...I expect the 100-400II to be very solid, very sharp, 4 stop IS, weather sealing, etc. They may have the size and weight advantage, but it will be interesting to see comparisons of the different 150-600's vs the 100-400II + 1.4TC.
Don't underestimate the value of that size and weight advantage. The Tamron and Sigma C both have 95mm filter threads. I expect the Canon to be 77mm or 82mm. I also expect it to be much shorter in overall length.
The Tamron is 10.1 inches long without the hood. It's 4.2 inches in diameter.
The 100-400L is 7.4 inches long without the hood. It's 3.6 inches in diameter.
The 70-300L is 5.6 inches long without the hood. It's 3.5 inches in diameter.
If the new one (if it exists) is really over 2 1/2" shorter and over half an inch smaller in diameter, that's a big difference in storage and usage.