Lenses built to be light weight and portable tend to be like that. It takes a lot of weight to adjust for sharp edges. I use the 21 ltd. for it's rendering in pastel daylight scenes. It doesn't handle contrast well, it's not edge to edge sharp. If that's what you bought it for you bought the wrong lens. Tha being said, at reduced size it's rendering is wonderful in 3D scenes with transitions and out of focus areas. I wouldn't even point mine at a brick wall. It would be an insult to the lens designers, in that, they didn't design it for brick walls. It's a "Lens designed for the way people take pictures, not for the test charts" lens, as are all the older limiteds.
The less weight you design a lens for, the more compromises you have to make.
That being said, the lens should have excellent center sharpness, with ƒ3.2 to ƒ8 being pretty much indistinguishable in the middle.