I use two sigma Art lenses and indeed they are very good...
But i am sorry to say that Dan Well's logic seems to be off a bit, now and then.
He shows a panorama of a sea scape with the text :
No distortion to speak of – this is a stitched panorama from about 8 shots – the original is 50,000 pixels wide… 85mm Art
first of all: a panorama is made by calculating and eliminating the lens distortion- so if there is any it shoud be gone.
second : i see a clear S-shaped horizon. So, a bad example in any respect. Also showing his panorama skills are not top notch.
( moving waves make it impossible to do a perfect stich anyway)