Jim,
Links to two reviews of the Sigma 20mm f1.8 that I have managed to find.
http://www.the-digital-picture.com/R...ns-Review.aspx
http://www.fredmiranda.com/reviews/s...hp?product=125
From those it appears wide open is soft and you may be at f4 for acceptable centre sharpness. For your off centre subjects then you may even have to stop down to f5.6.
I think you are in a try before you buy situation with this one.
Don