[Royl]

Well, that depends on the borders :-) A sharp center is no real achievement. 

If the borders/corners was as good as the center, it would be a 5* rating - in this case the other characteristics would be considered to be minor aspect but then no zoom lens on mother earth could do that.

As is the performance at 70mm is the dominant factor for the rating.

That's the awful truth. Most major builders have learned the lesson that center sharpness is king, as with the Sigma 17-50mm. They have to get that right, if nothing else. The 16-70 can produce some really nice results in the middle portion of the range, particularly stopped down a little. I like the lens. But PZ is right about sharpness at 70mm, and that destroys the value of the lens overall. Too bad.