[Brightcolours]

I'm not sure the 75-300 is even sellable... Like Brightcolours, I would probably recommend to use Tamron 70-300 VC, as it's better than the Canon 70-300 (and naturally the 75-300) mechanically, and surely optically as well.

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

The Sigma 8-16 may not work at all... I tried a 10-20/4-5.6 on my APS-H bodies during a recent trip, and not only it had black corners at 10mm, but also strong vignetting everywhere else, even at 20mm (and that on 1.3x crop, mind you!) If the 8-16 uses the same principle, it may not be work at any focal length on FF. But I've never handled that lens, so it's just a guess.

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

24-70 macro... funny, I've seen a guy using one on a 6D at the Venice's Biennale in early August. Though there aren't many tests of this lens around, it'll probably be okay'ish in the center, with pretty blurry corners. Here's one review, so you know what to expect:

http://www.the-digital-picture.com/Revie...eview.aspx

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