I agree that a lighter car is faster over a single lap but i think people add weight to make the car easier to drive, i personally never add weight , i like the car to feel sharp. (i did add 20g to the front of a losi cr though to make it steer

)
I know a british company actually had someone work with them from mclaren on this sort of stuff, it obviously cost them a fair bit of money and the result was, they learned nothing because RC is very different to 1:1 cars.
Just because the cars are 1/10th scale it doesnt mean gravity and air particles are 1/10th scale