Under no circumstances is an R/C car benefited by a heavier rotating mass. In fact, there is only one reason for tuning a 1:1 race car with flywheel weight- because the stored energy in a rotating flywheel is what the car uses to accelerate it from a standing start and on gear changes. Since an R/C car has no use for stored energy that would only hinder decleration, there is no reason to have it. That includes the pinion gear. Bottom line is: the more rotational unsprung weight you must turn, the slower the car will accelerate.