I have somewhat of a similar issue.
I own a Nikon D5000 with the 18-55mm VR and found some of my images were soft.
My local camera guy suggested that if I want consistant sharpness, that I need to upgrade to a better lens like the Sigma 17-50 f2.8 or pro Nikon brand but in either case, I am looking at upwards of $400 to spend.
The next day, he called to tell me that someone traded a mint Nikon 24-85mm VR, and he would be willing to sell it to me for $250. Knowing that I was on a limited budget, he told me that being an FX lens, sharpness would be better, and consistant in the center on a DX body so it would be a step up from my 18-55mm VR kit lens.
So would you agree with his recommendation that one could consider it a budget lens step up, and the price was right?
I bought the lens, and it does seem sharper to me. As a non-pro, sharpness and color rendition is more obvious to me than chromatic aberration and distortion which I do not really look for in my photos. Any my D5000 can correct for the distortion.
Elsewhere, I have read that in using an FX lens on a DX body renders corner shaprness issues moot.
What do you think?