SIGMA is very good at all focal lengths. I certainly do use it at 600mm.
No it is not as good as the 100-400 Mk2.
Is it good enough that it makes no difference in real use? Eye of the beholder I would say, but I was very happy with the mk1 100-400mm for a decade, and i do believe the SIGMA is as good or better than that lens.
How about the 100-400ii with the 1.4X lll? I lose more detail than I like. I'm guessing the Sigma is probably better than that..... but that's why I'm hesitating.