Is my 16F28 good, bad or average?

I got a 16mm pancake a few days ago and can't decide if I should keep it.

Having read many reviews and comments before, I knew that it could have decentering and corner sharpness issues and that it performs best somewhere between f5.6 and f8 at which point the camera bumps up the ISO quite a lot depending on the light conditions.

Mine is poor in the corners from f2.8 to f5.6, the blurriness being especially apparent in the lower right-hand corner ( not just extreme ), stopped down to f6.3 or f7.1 it's quite  good ( or at least acceptable to me ). The lower left hand corner is consistently better in every f stop then the right hand corner. I do not have the same problem with the kit 18-55mm lens, which at 18mm even at f3.5 is way better and there is no noticeable difference between left and right side.

I wanted to ask other 16mm owners if they think mine good, bad or exactly what I should be expecting? Should I try to exchange it? Are yours any better open at f2.8 or f4?

If I got the UWA converter, will the problem get worse or better? What is your experience?

I am posting a few handheld JPEG shots from f2.8 to f8 for comparison.

Can anyone post their examples at f2.8 and f4?

thanks

f2.8 worse than my P&S

f4 still bad

f8