I've owned both lens 15-85 and 17-55 for over a year each. The 17-55 is optically superior to the other zoom options, but I'm not sure why people automatically assume optical superiority should dictate the purchase, especially when none of the mentioned lens are slouches.
Purpose/function should be the determinant IMO...which puts the 15-85 ahead. Car shoots are controlled environments, usually during the day, and if at night using a tripod is simple since your subject can be stationary. The wide end of the 15mm can be useful for getting close to interesting parts of the vehicle for exaggerated geometry, and the zoom to 85 allows great flexibility to shoot from a variety of positions. Sure, sharpness might not be as good as the 17-55 when compared at 100% crop side-by-side, but let's be realistic, sharpness from this lens is a non-issue.
In a nutshell, any of the lens will be an upgrade, but you really should think beyond sharpness/contrast for the OP's described purposes. Not to mention, the 18-55 kit lens isn't exactly soft, and especially not when stopped down a bit...which I assume will/should occur frequently to keep the large subjects in focus.
