It could be that the proper focus point is beyond infinity on your particular camera, so no matter how much you compensate, you can never hit the true focus point.
I'm not offering any excuses, there's a great deal of variation with these cameras (shoddy quality control?) but I know what my lens can do wide-open, so I know it's a good lens. Some people have taken the cameras apart and re-shimmed them. Personally, I wouldn't waste the time.