A few other things to consider:
First, most UWAs respond nicely to being stopped down a bit - wide open they can be a little soft. ; Have you tried the same shot at F4, 8, and 12 for example, to see what effect it has on sharpness?
Second, consider the actual use you plan with the UWA. ; When I was shopping for one, I was looking at a Sony 11-18, a Sigma 10-20, and a Tamron 10-24. ; Overall it seemed the Sigma had better reviews maybe 75% of the time, and the Tamron maybe 25% of the time. ; Most of the criticism on the Sigma dealt with softness on center at full wide, and odd distortion on the horizontal plane. ; Most of the criticism of the Tamron dealt with corner softness wide open, and overall softness over 15mm. ; The choice for me became clear after testing them - the Tamron...because I very much intended to use this lens for the 10-14mm range, and could care less about anything beyond that mark. ; It had good on-center sharpness even wide open and good distortion and flare control. ; Any of those lenses have their strong suits, and all have some compromise areas. ; Just worry about the typical focal range and aperture you are most likely to be using the lens with most of the time, and if it does well at those settings, you're golden.
Third, don't forget how sensitive a UWA can be to recomposition after autofocusing. ; Because of the extreme curvature and ability to shoot objects in frame over such a wide focal plane, if you turn slightly to one side to focus on a near object, then recompose the shot with the object off center, the actual focal distance is adjusted due to the angle of approach, and can be off by a few feet. ; If you're shooting wide open, that could be enough to cause slight OOF blur. ; To test the lens, make sure you're shooting a subject straight on, and not recomposing. ; And consider either manually focusing or using the adjustable focus points when you want to take a shot with an off-center subject.
Just a few of the things I've been learning as I adjust to UWA shooting! ; It's a bunch of fun learning though.