I am not using a pair of 3 cells in series, I am running a single 6 cell lipo pack. I run batteries with a lesser capacity (2200mah and 3000mah) because there is not as large of an amp draw with a higher voltage, lower kv system than you have with a 4s setup. The single 6s lipo weighs less and is physically smaller than a 5000mah 4s battery. The idea is to get the lowest mah battery as you can that will still last the heat or main. This allows you to keep the weight down. The advantages of a lightweight vehicle boil down to pure physics. It takes less power to move an object of lesser mass (which will add run time and keep the heat down). The lighter car will also accelerate at a faster rate than a heavier car with the same force applied to it. Furthermore, the car will not have as much momentum (the formula for momentum is mass times acceleration) because the car will not have as much mass as a heavier car. This will allow the car to brake easier (which is easier on the esc, and keeps the heat down). This allows you to pick a braking point further from where you would have to with a heavier car which means you can carry higher speed for a longer period of time. This equates to faster lap times. I am a firm believer in a lighter vehicle. In my opinion, the pros far surpass the cons. There are those that argue that if you need to add weight to obtain the handling characteristics that you desire (a more planted car, better steering, etc), that your car was not setup properly in its lighter state. Through setup, those characteristics can be obtained with the lighter car along with the various other advantages.