In comparison with the Tamron 150-600 two things stand out:
Weight: 2860 g (vs Tamron's 1951 g)
Expected price: around $2,000 (vs Tamron's $1,069)
The Sigma better be better.
That seems a little unclear to me Paul unless I have missed something(very likely lol)- yes it will AF up to f8 (with the right Camera of course) but if the new tc is a 1.4x then that will take it over f8 at the long end!!! is it just going to AF up to 400mm (where assumable the f5.6 moves to f6.3) OR are they saying that the f6.3 + the 1.4x tc will only report f8 although it is more than that ?Sigma rumours announced 2 150-600 lenses a sports version that will use the dock and a contemporary lens both will AF at F8 with the new TCs.