My sample is definitely soft but improves until I get optimum sharpness around f/16. Tested on a tripod (however only tested at 200mm... other focal lengths might show improvement).
However... why buy a f/2.8 lens to use at f/16, esp. as many people shoot at the 200 end most of the time.
However it was cheap (Used) and much less bulk than the Canon equivalent.
An acceptable lens but not great. I may have dud sample...