I'm not familiar with that Canon lens, WW, but I'm very
impressed with the Sigma.
I have a really, really
good 100-400mm, and the Sigma outdoes it in every metric: the images are sharper, the AF is quicker, the stabilisation is brilliant
, and - a surprise I'm really pleased about - the colour and contrast from the Sigma really are obviously
better than the 100-400mm - that was the very first thing I noticed about the files.
It's damn' good with a TC too. This
is 420mm with a Canon 1.4x (and a heavy crop) and it's good and sharp
with two stacked 1.4s as well.
These are both "wide open". A key reason for me buying it was the additional DoF flexibility provided by f/2.8 if I want it, and it is definitely very usable wide open. It gets silly
-sharp by f/5 (assuming - as will usually be the case - that I'm starting at f/4 because of a TC).
I only ever shoot handheld, incidentally: it's a heavy lens, but I'm already used to it.
I'd always said that regardless of what I upgraded to, I'd never part with my 100-400mm: now, I'm really not so sure about that.
I recently read a comprehensive review
of the new Canon 300mm f/2.8, and - based on the sample images - the Sigma is barely a hair behind in sharpness terms.