[Rover]

The Tamron is better than the 75-300 (I tried the latter and it was no good above ~150mm). I might've not phrased it clear enough.

The Tamron is not better than the Canon, optically. They are more or less equal.

The Sigma 8-16mm is vignetting free at 15-16mm, on FF.

That is not the lens the original poster has. He does state he has a 24-70mm macro version.

Huh? The review I posted the link to is for the old Sigma 24-70 macro, not the HSM one. I assumed that was the lens the OP was talking about. Quote from the review: "With a minimum focusing distance of 15.7" (40 cm), the Sigma 24-70 has a maximum magnification of .26x. This is enough magnification for Sigma to give this lens a "macro" designation."