The tuning options in GT5 are not very true to life due to their nature being quick tuning. Being realistic never was the idea of this. You would be able to reach those numbers in reality for many of the cars though, at least in theory.
Still, you can make a car lighter by throwing out things you don't need for racing, such as carpets, rear seats, spare wheel, tools, changing body panels for lighter versions, thinner glass, and so on...