My suspicion is that when Canon announces an updated 100-400 it will handily beat the Tamron where the ranges overlap and that cropping it will come close to the tamron at 600.... But I'll bet that if/when an updated 400F5.6 comes out it will blow the works away for IQ....
One of the things I like about the Tamron is the zoom range... Two of the most popular L glass lenses are the 28-70 and the 70-200's... a 100-400 has an awful lot of overlap.... the 150-600 does not. I'd like to see Canon change the range of the 100-400 to something like 200-500 and with the zoom range being reduced from 4X to 2.5X there could be a huge jump in IQ.
200-500 would be perfect!
Mine ships on Monday, and should be here by Wednesday! I've been very happy with my 75-300L, but more reach is needed and I'm not going to reward Canon for complacency. How many years have there been rumors about a 100-400 replacement?
I look at the Tamron as a lens to get by with until this phantom mkII 100-400 arrives when were all 150 years old!
Will have four days off to test it out on the 5dmkiii and 70d