"Maybe you ask to much of a zoom lens in this respect.
Zoomlenses i am familiar with always corner problems, even fixed focal lenses.
yes, a perfect image you may expect from the new Nikon 180-400, but that costs..."
But that's the whole purpose of the LR/ACR Lens Profile tools! You can take a mediocre lens and make it perform like a much more expensive lens. This is true for Distortion, Vignetting, Chromatic Aberration, and in many cases image sharpness fall-off. The latter using a Radial Filter that applies graduated Sharpening to the lower resolution edge and corner areas. You then create Develop presets to apply these corrections on Import. EZ PZ!