When you loose weight the car become heavier, as the total weight of the car has to be 600KG with the driver. So if the driver loses weight, the car has to gain it back again. In this case, they place the weight lower in the car to make it go faster. But it's not that big of a difference overall.
current f1 cars weight abt 550kg.
if the drivers is lighter then the engineers has a bit more ballast to play with, which might help the car goes a few hundredth or thousandth faster.
Points are not really a true indication of whats fastest, like look at Williams there 4th but in terms of pace i would say there equal with Honda, and Renualt are 7th when they are about equal 4th with Redbull and Toyota.
Force india are faster then Torro rosso but just behind Honda in terms of pace.