I too have a Sigma 600 for a contax mount. I use it on safari and let the automatic shutter speed determine the exposure. This worked well to late evening until the speed dropped to below 1/500. bean bags were a great help. Doughnuts are a problem only when there are specular highlights; water and man made sufraces. No such issues in the grasslands of east Africa.
One other advantqage is the slightly lower contrast of the lens with the central obstruction of the reflecting mirror, under high contrast situations.
Your best lens is the one you got.
Hand held, $90, short, light, built in B&W filters, easy focus which snaps in and out of focus
For me it was a great choice