Morning and welcome to the Forums!
I do not have the lens, however several reviews have the lens soft at the edges and in particular in the corners.
So if you jump to the second page of this review, you will see a see a section titled "MTF (resolution)" with a number of histograms (charts). What these charts tell you is that the lens center is very good, and you can see at the various focal lengths, as the lens is stopped down from f2.8 up through f8 that resolution improves both in the center and the edges and corners. So for each of the focal lengths you want to find the f stop (aperture) where the center has a reasonably high value, along with both the edges and corners. You might have to give up some sharpness in the center in order to pull up the edges and corners. So it looks like at 28mm its f8, at 40mm its f8 and at 75mm it f8 also.
Take a look at Part II Performance section of this review - down at the bottom it walks you through the lens and how it performs.
Most lenses are not at their peak performance at the extremes, so stopping down is very normal.
Also, I don't use Lightroom (yet, I am thinking about changing), but have you selected the image you want to delete and hit the [delete] key?