I used to have the Tamron 150-600mm, and it wasn't as good as the 100-400mm II + 1.4xTC. I recently compared the Sigma C and Sigma S 150-600mm at 600mm with the 100-400mm II + TC and found the Sigmas to be excellent and of similar IQ and resolution. On the other hand, the Canon 100-400mm II appears to be very consistent and is a safer bet to buy unseen. According to lensrentals latest spurt of testing lens variation, the new Canon lenses are exceptionally consistent.
Thanks again: My buddy did let me shoot the 100-400 II w/o the tc, on his 7D II before I bought mine, and I was wowed by the iq - of course the birds were close enough that the tc wasn't really needed, but my eyes were opened. Of course, that also puts me in a position to have to save up for even more bucks to get the 100-400 AND the tc, but it'll also be a good way to curb my impulsiveness around new camera stuff....
7D II, ef100-400 II:
6D, 7D,50D, 17-55IS, 24-105L, 70-200 Fl2.8L IS, 135L, tokina 16-28, 150-500 & a buffet more, 580 EXII, 430 EXII, AB 800's