Also don't forget that carbon fibre is electrically conductive, if there is a lot of static in the air (in LFS this happens a lot when passing those wind turbines at Blackwood) this can result in your car becoming positively charged. If the polarity of the charge is correct then you'll storm up the race order, but if it's the reverse polarity you're in big trouble.
Ah yes one more thing, when running a light car obviously your data packets to the server are lighter too. If too many people in the same race are using a light car then it can effect the weight of the entire internet.
Infact wasn't there that "Stock Lites" league that caused the server hosting the race to loose so much weight that midway through the championship finale an engineer had to treat the race server for anorexia which gave us that highly controversial finish to the series?