Basically, Tris has it right. Or at least partially right. All road cars are, of course, built to a market price point and therefore a set of compromises. As they come out of the factory they are the best that they can be for the price point and market position intended by the manufacturer and the idea that spending a few hundred pounds on body kit, loud exhaust and/or larger (read heavier) wheels in isolation is going to make the car perform better is just ignorance pure and simple.
However, that doesn't mean that such a car can't be improved to meet the individual users set of compromises, (which may well not be the same as the manufacturers), to improve certain aspects of the cars performance. But, it requires a proper understanding of vehicle dynamics and comprehensive set of changes to improve things such as suspension performance. These things aren't actually difficult to understand, but most people never bother to take the time to understand them before embarking on their £££ spending spree on bigger heavier alloys, plastic bodywork etc.