Any lens made by nikon or canon these days is going to be damn near perfect for any practical usage in terms of sharpness. Don't worry about sharpness unless you yourself consistently notice it in your images without looking for it.
The only reason the kit lenses are not wonderful and magical in every way is for the reasons that are clearly advertised right on the lens: their speed (usually fairly slow, 3.5 or higher), and their lack of ability to go to extreme focal lengths in any direction. And lack of build quality usually, and lack of fancier features like USM or stepper motors, etc.
Upgrading generally does things like giving you a wider range (if you want a zoom still), OR giving you a brighter lens (with the same zoom or even more so for a prime), or a quieter or faster autofocus, or sturdier, or whatever. Nothing's really being hidden from you, and I don't think it's worthwhile to worry much about any non-advertised numbers for these companies (like sharpness). Just assume everything will be more or less perfectly sharp unless proven otherwise.