There are so many variables here, it would be impossible to give you a precise answer. It depends entirely what you are wanting to do...

The kit lens can, in many cases, give a satisfactory photograph, however there are two main areas in which the kit lens suffers in competition against a pro lens:

1. Aperture. Kit lenses are slow. They are usually about f/3.5 on the wide end and by the long end have stopped down to f/5.6 or so. This means that it makes your job a little harder to get a good photograph in say low lighting, indoors, or in the dark. It also means you can't open wide enough to get that shallow depth of field which helps make some photos.

2. Lens quality. As kit lenses are so mass produced, they do not get subjected to the same rigorous quality checks that pro lenses do. The design too, means a cheap lens which whilst perfectly usable, has certain flaws. These will be in the areas of things like:

   • Sharpness
   • Chromatic Abberation
   • Flare
   • Distortion
   • Robustness (eg. build quality - it will take the knocks, a bit of rain won't hurt it, etc).

You will find a pro lens design helps to reduce all these things, whether it be clever design and months and months of curing the glass and polishing it to within an inch of its life, to the coatings that they put on them. You will find that a pro lens is generally much sharper towards the edges of the frame, and that things like barrel distortion is greatly reduced (except on say, a fish eye where it is actually a desired effect).

If you took your 18-55 kit lens, set it to 55mm, stopped down to f/8, and took a "pro" lens such as the EF 24-70 f/2.8L USM, and ALSO set it to about 55mm, f/8, and took two identical photos of the same thing, with the same lens with the same camera settings, you would I have no doubt see a very similar image. However they won't be quite the same. You will find the sharpness of the L lens better than the kit lens. You may find the colours are richer. If there is a light source such as the sun, or a light, you may find the kit lens exhibits more flare than the L lens. The edges of the frame should be sharper with the L lens too, and you'll have less barrel distortion. Looking at areas of high contrast such as the edge of a building and the kit lens will be much more susceptable to things like chromatic abberation which appears as a green tinge along one side of the area of high contrast, and a reddish tinge along an opposite area. The pro lens should handle this better, if not eliminate it (though its sometimes near impossible).

All that said -- post production software like Adobe Lightroom is getting better all the time. I notice it has options for things like Lens Correction Profiles, and "Removal" of chromatic abberations. How effective that is I'm not sure but it all helps.

So.....yes. Difficult to say really. The cheap kit lens starts way behind the pro series lens for all those reasons and maybe more. But if you like the photo you took with it, and process it well in post - why not hold it up alongside a photo taken with a Pro lens?

Besides - YOU are the magic ingredient in your photography. A better photo will be taken by a skilled photographer with cheap kit, than a total novice who doesn't know what they are doing even if they do have fantastic uber expensive kit!